I think it is very unlikely that its is a CPU cooling problem, unless he is using a very old Athlon. I can start my pentium 4 (478) and get into windows without my CPU heatsink and fan . When you tried the other graphics card did you disable onboard video? do you see any booting up at all? if so what stage? If it was a memory or graphics failure you wouldnt even get the bios loading beep. My 9800 Pro failed a few months back and the computer wouldnt boot.