Look at your total and available physical memory. The difference between the 2 is what's being use.
If you think your system is slow, 1. buy another 1gig of RAM for it. 2. do a full scan for spyware/viruses 3. run a hard drive defrag.
Make sure you don't have 20 things running at startup and clogging up your system tray by the clock, and have several different anti-virus software programs running at once. I have seen computers where people have installed 3 A/V programs, not very good for performance.
If those things look good, buy a new drive and re-install Windows on it. Keep your original drive as a file back-up.