I have had this a number of times with different PC's here at work and once on my home one. I would also like to say there hasnt been a single fault the same every time.

It has been the monitor (BenQ 17" flat screen) twice, most commonly it is the graphics card slot on the motherboard that has blown, it has also been down to incompatible RAM or in the wrong slot(s).

I have one next to me at the moment with the same problem which is our accountants home PC and it looks to be the actual graphics card this time. The worst part for me is 4 and a bit years ago 60 PC's were purchased including ones for some senior staff, they are all now falling over inc the Servers