Sounds like one of two things...
Corrupted windows files (boot, reg, drivers)
Failed hardware
For the windows stuff, drop in your os disk and see if you can get it to repair (or restore if you have that turned on)
If that doesnt work, spend $20.00 and get a usb external adapter for internal had drives and pull your files/info off the original disk and then reformat/rebuild.
For hardware, pull EVERYTHING and I mean EVERYTHING off the mainboard, boot it without mouse/speakers/HD/ram/sound/video(use buildt in, if not avialable use your current video card) if its stable in the bios (the only thing you can get into like this) then start adding items one at a time starting with video, then ram, then hd, and so on.
PS. I feel your frustration I just rebuilt my machine swapping from xp X64 to vista business, now i have some items on my list..
