I had the same thing happen last year when I built a PC. It turned out to be a bad MOBO, and I had to RMA it, twice
If you have spare parts(PSU/RAM) laying around try swapping them just to make sure it isn't one of those things.
Just out of curiosity.... Did you get a new hard drive? Or did you just try using the old one without a fresh format? Well, I guess it doesn't matter because you should still be able to access your BIOS.