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.

The best resource for this kind of thing is Tom's Hardware forums. Someone has almost certainly had your exact problem and posted a solution. The community there is really knowledgeable and helpful.

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.