My computer, just very recently, has begun to run at CPU usages of 99-100% regardless of what is running, even though according to Task Manager, I have barely 100MB of processes running - usually Mozilla Firefox, dwm.exe and explorer.exe, none of which are harmful. The others are little essentials like the DivX update, or graphics card driver addons. I consider myself to be a secure user, and am therefore completely unaware of how this could have happened.. I suspect that it is a malware/spyware infection that is eating my CPU, but after having run Spybot S&D and Avast, all I got back was one false positive.
Any help would be appreciated.