I'd check Device Manager for any yellow exclamation marks. That's Windows telling you a driver is missing or broken. Right-click My Computer, Properties, Hardware tab, Device Manager. Fix anything with a yellow mark and a lot of weird problems clear up.

Honestly at some point a clean reinstall of Windows fixes more than any amount of tinkering. XP gets crufty after a couple years. Back up your files, find your product key, and do a fresh install. It'll feel like a brand new machine.

Check how much free space is on your hard drive. Windows needs at least 15% free to run well — it uses that space for virtual memory and the swap file. If your drive is 95% full, that alone will make everything crawl.

This is a case where spending a little more upfront saves you money in the long run. Cheap hardware fails faster and costs more to replace. Buy quality components from reputable brands and they'll last years longer.

Before you do anything drastic, try booting into Safe Mode (tap F8 when the computer starts up). If the problem goes away in Safe Mode it's a software or driver issue, not hardware. That one trick narrows down half of all PC problems.

Definitely back up your important files before trying any fixes. I learned this the hard way when I lost 3 years of photos trying to fix a simple problem. Get an external hard drive or burn DVDs of your important stuff first.