Girando un pò in rete.....Prova a fare scansioni antivirus, fai girare Spybot e vedi se trova qualcosa.....
Ti allego anche qualcosa di simile al tuo problema trovato su un altro forum:

If Windows Task Manager shows System Idle Process at 100% (with CPU at 100%), it can also indicate program conflicts, corrupted system or program files, and bad or corrupted drivers. It doesn't necessarily mean you have malware, a virus, worm or Trojan. You could see if you have installed anything recently and try uninstalling to determine if that is the problem. Also, try individually stopping any applications that run to see which, if any, may be causing the problem. Be very careful stopping processes, though, as this may just lead to more problems.

al peggio prova a postare un log di hijackthis nella sezione sicurezza informatica e virus.
In bocca al lupo