How to Fix a Computer: Diagnose and Solve Common Issues

TL;DR: Most computer problems, slow performance, random crashes, won't boot, come down to a handful of root causes. Work through them in order, don't skip steps, and you'll fix the problem or at least figure out exactly what needs fixing. If you get stuck, we're here.

What You Need

Before you touch anything, gather a few things. You don't need a toolbox full of exotic gear for most of this.

• A working internet connection (on your phone if your PC is dead)
• A USB flash drive, 8GB or bigger, formatted and empty
• Access to another computer if yours won't boot
• Your Windows or macOS login credentials
• About an hour of patience (more if the problem is hardware)

That's it. The rest is method.

Step 1: Write Down Exactly What the Computer Is Doing

This sounds obvious. Nobody does it. They come in here and say "it's just slow" or "it keeps freezing" and that tells me almost nothing.

Before you do anything else, answer these questions in writing:

• What exactly happens, and when? Is it on startup, during specific programs, randomly?
• Are there any error messages? Write them down word for word.
• Did anything change recently? Windows update, new software, someone else used the machine, power surge?
• How long has this been happening?

Symptoms point to causes. A computer that freezes two minutes after boot is a different problem than one that freezes only when you open Chrome. Don't skip this step.

Step 2: Restart and Check for Updates

I know you've heard this a thousand times. There's a reason. A clean restart clears temporary files, flushes memory leaks, and applies pending patches that can cause all kinds of weird behavior when left hanging.

Do a full shutdown, not sleep or hibernate. Wait ten seconds. Power back on.

While you're at it:

• Windows: Go to Settings, Windows Update, and install anything pending. Restart again after.
• Mac: Apple menu, System Settings, General, Software Update.

If your problem disappears after this, great. If it comes back, keep going.

Step 3: Check How Full Your Storage Is

A drive that's more than 85 to 90 percent full will make Windows act like it's running through mud. macOS starts complaining earlier than that.

• Windows: Open File Explorer, right-click your C: drive, Properties. Check the pie chart.
• Mac: Apple menu, About This Mac, More Info, Storage.

If you're close to full, that's likely your performance problem. Clear out the Downloads folder, empty the Recycle Bin, and uninstall programs you don't use. Windows also has a built-in Disk Cleanup tool that handles temp files.

Don't let anyone sell you a "PC cleaner" app to do this. Most of them are junk. (Some are worse than junk. They're scams.)

Step 4: Run a Malware Scan

Not a "tune-up" tool. An actual malware scanner.

Windows Defender, built into Windows 10 and 11, is genuinely decent. Open Windows Security from the Start menu, go to Virus and Threat Protection, and run a full scan. Not a quick scan. A full scan.

If you want a second opinion, Malwarebytes offers a free version that catches things Defender sometimes misses. Run both.

Signs you might have malware: browser redirects, pop-ups you can't close, programs you didn't install, performance that tanks out of nowhere, or your antivirus has mysteriously turned itself off.

If the scan finds something and can't remove it, that's when you call a professional. Some infections require offline removal tools or a full OS reinstall to clean properly. We handle that at our computer repair shop.

Computer acting up? Get a real diagnosis. Book a free diagnostic

Step 5: Test Your RAM

Bad RAM causes some of the weirdest symptoms in the book. Random blue screens (BSODs) with different error codes each time. Programs crashing with no clear pattern. Files getting corrupted for no reason.

Windows: Search for "Windows Memory Diagnostic" in the Start menu. Run it. The computer will restart, run the test, and report back. If it finds errors, you have a RAM problem.

Mac: Apple runs its own hardware diagnostics. Shut down, then hold the D key while powering on. Follow the prompts.

If you have multiple RAM sticks, you can also test them one at a time by removing all but one and seeing if the problem goes away. That's slower but more thorough than the built-in test.

Step 6: Check for Overheating

Heat is the silent killer of computer hardware. A CPU that gets too hot will throttle itself down to avoid damage, which looks exactly like sudden slowdowns. If it gets hot enough, it shuts off completely.

Download HWMonitor (free, reputable) and watch your CPU and GPU temperatures while the computer is under load. General guidelines:

• CPU at idle: under 50°C is fine
• CPU under load: under 85°C is acceptable, over 90°C is a problem
• GPU under load: varies by model, but over 95°C is too hot

If your temps are high, the fix might be as simple as blowing dust out of the vents with a can of compressed air. A clogged heatsink fan is incredibly common, especially on laptops that have been around for a few years.

If cleaning doesn't help, the thermal paste between the CPU and heatsink may have dried out. Replacing it is cheap but fiddly. Don't improvise with toothpaste. (Yes, people try this. No, it doesn't work.)

For laptop repair, thermal paste replacement and cleaning are things we do regularly.

Step 7: Diagnose a Computer That Won't Boot

This one scares people. It shouldn't, necessarily. "Won't boot" covers a lot of ground.

If you see a black screen with nothing: Check the monitor cable. Try a different cable or port. Make sure the monitor is on and set to the right input. This sounds dumb until you realize how often it's the actual problem.

If you see a spinning circle or loading screen that never finishes: Force shut down by holding the power button. Try again. Windows will sometimes offer Startup Repair automatically after a failed boot. Let it run.

If you get a specific error like "No Boot Device Found" or "BOOTMGR is missing": This usually means the drive isn't being found or the boot partition is damaged. This is where you need a Windows installation USB. Boot from the USB, go to Repair Your Computer, Troubleshoot, Advanced Options, and try Startup Repair or the Command Prompt for repair commands like bootrec /fixmbr.

If you hear clicking from the hard drive: Stop. Do not keep restarting. A clicking drive is a dying drive, and every extra boot cycle risks more data loss. Get it to a professional immediately. If you have a backup, you're in good shape. If you don't, check our guide on backups and disaster recovery before this happens to you.

Mac won't boot: Hold Command-R at startup to enter Recovery Mode. You can run Disk Utility from there to repair the drive. If that doesn't fix it, Mac repair is what you need.

Step 8: Reinstall Windows (Last Resort for Software Problems)

If you've gone through every step above and the machine is still misbehaving, and you're confident the hardware is fine, a clean Windows reinstall fixes a lot of stubborn software problems.

Windows 10 and 11 have a built-in reset option. Settings, System, Recovery, Reset this PC. You can choose to keep your files or remove everything.

Back up first. Always. Even the "keep my files" option has been known to eat things.

If you can't get into Windows at all, you'll need a bootable USB made from the Microsoft Media Creation Tool. Download it on another computer, create the USB, boot from it, and follow the prompts.

If any of this feels like more than you want to handle, remote support can walk you through it live without you having to bring the machine anywhere.

Common Mistakes

Replacing hardware before diagnosing software. Bought a new RAM stick when the real problem was a corrupted Windows update. Happens constantly.

Clicking "Fix All" in some random optimization app. Those tools range from useless to actively harmful. Windows has everything you need built in.

Ignoring SMART warnings from your hard drive. Your drive has been telling you it's dying for weeks. You clicked through the notification. Now you're crying over lost photos. Run CrystalDiskInfo on Windows to read your drive's health status.

Skipping backups because nothing bad has happened yet. Something bad is going to happen. Drives fail. Ransomware exists. Laptops get dropped. The backup conversation should happen before the emergency, not during it.

Trying to fix a business machine the same way you'd fix a home PC. Business environments have domain controllers, group policies, and other layers that change the rules. If you're dealing with a work computer, talk to whoever handles your business IT before you start pulling things apart.

Panicking and buying a new computer. Most machines that seem "dead" are fixable for a fraction of the cost of a new one. Get a diagnosis before you give up on it.

Bottom Line

Knowing how to fix a computer is mostly about working through a logical sequence and not jumping to conclusions. Slow machine, check storage and temps first. Random crashes, check RAM and look for BSOD codes. Won't boot, don't assume the worst until you've ruled out the simple stuff.

Most of what goes wrong with computers is fixable. The ones that aren't are usually giving you plenty of warning signs before they die completely.

If you've worked through this whole guide and you're still stuck, or if you found a hardware problem you don't want to tackle yourself, schedule a repair and let us take a look. We're in West Palm Beach and we've seen pretty much everything.

Computer acting up? Get a real diagnosis.

Fix My PC Store has repaired thousands of machines across West Palm Beach. Free diagnostics, honest pricing, no upsell games.

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Frequently asked questions

What is the first thing I should do when my computer stops working?

Do a full shutdown, wait ten seconds, and power back on. Then check for pending Windows or macOS updates and install them. A surprising number of problems are caused by pending patches or memory issues that a clean restart clears up. If the problem persists after restarting and updating, start working through hardware and software diagnostics.

How do I fix a computer that won't turn on or boot?

First check the obvious: power cable, monitor connection, and that the monitor is on the right input. If the machine powers on but won't reach Windows, try letting Windows Startup Repair run automatically, or boot from a Windows USB and use the built-in recovery tools. If you hear clicking from the hard drive, stop restarting immediately and get it to a professional to avoid further data loss.

How can I tell if my computer problem is hardware or software?

Software problems usually show up as crashes within the operating system, slowdowns that got worse over time, or specific error messages tied to programs. Hardware problems tend to produce random blue screens with different error codes, physical noise from drives or fans, visible overheating, or a machine that won't post at all. Running a Windows Memory Diagnostic and checking drive health with a SMART tool helps narrow it down quickly.

Is it worth fixing an old computer or should I just buy a new one?

It depends on what's wrong. A failing hard drive, bad RAM stick, or overheating issue can often be fixed for well under a hundred dollars and give you years more life from a machine. If the motherboard or CPU has failed on a machine that's already six or seven years old, replacement often makes more sense. Get a diagnosis first before assuming the machine is done.

Can I fix my computer remotely without bringing it into a shop?

Yes, for most software problems. Malware removal, Windows errors, slowdowns, configuration issues, and boot problems that still let you reach the desktop can often be resolved through remote support. Hardware problems like a failing drive or dead RAM obviously require hands-on work, but remote diagnosis can at least tell you what you're dealing with before you bring it in.

Why does my computer keep freezing randomly?

Random freezing usually points to one of four things: overheating, failing RAM, a drive that's nearly full or starting to fail, or malware running in the background. Check your CPU temperatures under load, run Windows Memory Diagnostic, verify your storage has at least 10 to 15 percent free space, and run a full malware scan. If all of those come back clean and the problem continues, the issue may be a driver conflict or a failing hardware component.