I came home yesterday to find my computer was turned off, and wasn't starting. I went to the store and bought a 450w power supply, still wouldn't turn on. I went back and bought a 500w coolermaster, and now it turns on but it flashes a blue screen really fast and restarts itself. After I kept checking the blue screen over and over again I found out it's error code "Stop 0x0000007B".
I tried swapping different HD's, RAM, Re plugged everything in, and still getting blue screen flash and restart.
The only option I can think of is that I still don't have a high enough power supply, which doesn't make since because the one I had before that ran for a while was only a 450w.
CPU: Intel Dual Core @ 2.3 Ghz
GPU: Nvidia GTX 550 Ti
OS: Windows XP Pro
according to the website you only need a 400W power supply, and that should be a generous rating. Did you reset BIOS after installing the new power supply? If the old supply died then it is quite likely the motherboard is dead as well. And the only fix for that is a replacement board, or an inconveniently timed upgrade.
This error is related to an inaccessible boot device. Could be drivers, virus, hardware conflicts. You could try doing a repair install of XP. If that works, then you could backup any files that you really want. I'd consider a format/reinstall.