Ok, I have seen similar problems like this, but mine has quickly escelated. Twoweeks ago I swapped out a hard drive and added a new one. I also installed a fresh copy of Vista Home Premium ( A legal one).
After about a week, it started shutting down right after Vista's desktop loaded. I cleaned out the heatsink, and switched out RAM. No go. So I order a new PS. My old one was less than a year old, but was running a sound card, video card, and 4 hard drives (with the 4th added when I redid everything). It was only 500 watts, so I figured it must be too small to run all of that. The new one I ordered is 680 watts.
I got it this afternoon and installed it. It did the exact same thing so I restarted in Safe Mode, and that seemed to work. So I restarted again to try regular startup and it started turning off before Vista would fully load up.
I checked the web and someone mentioned reseating the video card. So, I reseated the Video and when I went to turn it back on, the screen flashes for about a second, then goes black. No post. The fans still run, but it doesn't sound like the HDs are running. I can open the Blu-Ray drive tray so that's getting power...but now I can't even get into the bios!
The only other thing different (except of course the new OS install) was t hat I took a 750 gig HD that was a WD My Book external drive, and installed it in place of a 500 gig HD that was in there. Could it be something to do with putting the external drive in my PC?

Any help is appreciated!