Shoutbox

hi, i am having a problem with my computer , it keeps restarting by itself, at first it would let me into windows i would be able to use the computer then it would restart, now when i turn on my computer it boots up, i hear the CPU fan going for about 5 seconds then it turns off.

EDIT: i have tried everything, i cleaned out the CPU fan, and the rest of the fans, i left the tower open and tried it, nothing, i tried it with a fan blowing air into the computer and still nothing.