I had this problem before, My cpu is running at 100% and I dont know why. When I do ctrl Alt delete nothing under processes is running high. The highest is iexplore.exe. When I click the "Performance" tab and click "Resource Monitor" I can monitor my cpu and it shows "system "PID 4" "Threads 105" "cpu 95% avg", so the "System" is running at 95-99% avg. This all started when I was trying to boot unreal tournment 3 and it wasnt booting, so I had to restart comp. Running vista 64bit New hard drive 6400 amd cpu Also I cant do end task on it and restarted comp or running in safe mode dosnt help. No virus also. I have fixed this problem before by installing windows on a diferent hard drive.