CPU cycles near 100% - CPU usuage slows down the computer. See what is causing it.

Open Task Manager (Ctrl-Alt-Del), look at the Processes tab: Click on the CPU heading there to sort the processes according to how much CPU usage they're getting. There, you'll see exactly whatever's consuming your CPU time.
Take your time and watch the ones using the highest amount.
Once you know what process or processes are keeping the CPU so busy, remember the image name(s) If the listed processes are unknown, search Google on the process name to help identify it, or what software "owns" the process.
 http://www.google.com/search?q=identify+process+xp
If it is not critical, highlight it and click End Process and uninstall the control program if not needed.