PC-Booting Problem

Hi everyone,
I am running a dual boot system with Windows XP and Vista. The computer works well for a couple of days. This morning I could not boot my computer! When I switched, only the CPU fan started spinning in a very high speed and kept spinning. No beeps, no display on the monitor. I thought it was the problem of PSU. I put another good one in to replace it, but no luck. I took one of the two memory cards out and tested it. No luck. Took the other out and put the first one back in and tested it. No luck. When I was playing with it, the computer booted up once and took me to the operating system selection screen. I selected Vista and then it gave me a blue screen... I don't think it is a video card problem, because the fan should not spin like that. Any idea could be helpful. Thanks a lot in advance.