Hey...
So, obviously you have a hardware issue, and there is only one way to sort it out, through process of elimination... this is sometimes difficult if you don't have a store of the same parts lying around to test it with.
Anyway you can start with what you have but I recommend taking it back to your PC vendor so they can sort the problem out for you.
1) Unplug the dvd drive, and test...
2) If possible, find another HD and reload windows on it and test...
3) If you have two ram modulles remove one, test, swop them, test...
4) It could also be the display card so see if you can loan one and test your pc with it...
5) Remove all pci cards and test
The idea here is to test them individualy and see if there isn't a change after doing one thing. I suspect it could be your motherboard, but for you to test that you would need a professional enviroment with stock to test it with, like a PC shop, take it back and let somebody else sort your problem out...
Good luck....