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.

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.

I always tell people: buy a UPS battery backup. A $40 one will protect your PC from power surges and brownouts that slowly kill your hardware. Lost a motherboard to a thunderstorm once. Never again.

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.

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.

I build computers for a living and can tell you that most of these problems come down to three things: not enough RAM, outdated drivers, or malware.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