CPU usage at 100% when I use Firefox?

I would doubt very much if Mozilla is the cause of your problem, it uses less resources than IE. Chrome from Google uses even less resources in its standard form. I think, if you haven't tried HJT yet you should. They will yse your computer from an auto log and see what is actually running. Have you tried starting up in "Safe mode" and see if it still goes at 100 percent. You may still have a rogue process and a restricted boot load may help identify it. My 7750 runs at 2 percent as well, and that isnt a modern CPU really, Two Pentium cores I think. For XP, 512 is about enough for most things