CPU @ 100%, slowing everything to a crawl

We have one computer that for the last month has been using 100% of the CPU. When you try to open a program it just crawls it’s so slow & it’s hard to do anything with it. We have Trend Micro for our virus software & it does spyware scans & we also have run AdAware on it & only found a few items. What could be causing this & how do we fix it. We have Windows XP PRO & use Firefox for our browser. Any suggestions would be welcome.