What type of enviroment are you in? If you have the space that is remotly dust free, and (more importantly) static free, set the computer up with jsut the Power supply, mobo, CPU, ram, hdd, and vid (I think thats it) when I would work on computers at a store (NOT BB) I used to run into celerons that when you pulled it out of the case and tested one thing at a time it would work.
Have you tested the power supply?
Just a few ideas
Ollie