What is the error message?
Was windows 7 running on it well before?
Hard problems are shown by pc not going or even getting to bios stage. So all the fans might be spinning, given its not a motherboard or power supply problem, but nothing will show on the screen.
Software problems are shown by the pc being able to pass bios bt sometimes hangs, or a error is shown like system32 file missing.
If by any chance your mbr is infected by virus like revolver, it goes into a loop of rebooting have way through before you get or soon after login.
