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.

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.

Have you run a full virus scan AND a spyware scan? They're different things. Norton or McAfee for viruses, plus Ad-Aware AND Spybot for spyware. Run all of them. A slow computer is infected until proven otherwise in my experience.

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.

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.