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.

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.

Newegg is your friend for parts. Way cheaper than Best Buy or CompUSA and the customer reviews actually help you avoid junk. Just watch out for shipping on heavy stuff like CRT monitors. Read the reviews before you buy ANYTHING.

I'd post this on the Tom's Hardware or AnandTech forums with your full system specs — CPU, RAM, motherboard, the works. People can't really help without knowing what you've got. The more detail you give, the better the answers.

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.

Don't forget about heat. If your computer is shutting down or crashing randomly, open the case and check if the fans are caked with dust. Blow it out with a can of compressed air. Overheating causes a shocking number of 'mystery' problems.