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.

Make sure you have the latest drivers from the actual manufacturer's website, not whatever Windows Update gives you. Video card drivers especially — nVidia and ATI release new ones every month and they make a real difference.

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.

I build computers for a living and can tell you that most of these problems come down to three things: not enough RAM, outdated drivers, or malware. Check those first before spending money on new hardware.

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.