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.

I build computers for a living and can tell you that most of these problems come down to three things: not enough RAM, outdated drivers, or malware.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.

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.

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.

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.

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.