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.

I build computers for a living and can tell you that most of these problems come down to three things: not enough RAM, outdated drivers, or malware.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

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'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.