A friend's computer has started freezing. He asked me to have a look at it. I started it, Windows 7 lead me to a screen where an option to do a system restore was displayed and selected (there was an option to boot normally). I let it do the restore, especially since it wasn't responding to the keyboard at the time, and it did so and loaded fine. Avast was malfunctioning, so I re-installed it. All seemed OK. Then, after awhile, it froze again. I did a reset, and it won't boot - the screen was blank. I decided, just as a test, to switch the screen (had a spare) and the keyboard with different ones that I knew were working. Booted (all worked). Then I ran MemTest, 5 passes, no errors. Rebooting after MemTest the computer stopped at the "detecting IDE" phase of POST, and became irresponsive. I tried several more times (doing a cold reset each time). Once it booted into Windows, then froze.
The computer has a Gygabyte motherboard (I forgot the brand) with on-board video (no video card) and an Intel Quad CPU.
My question: is it the CPU of the motherboard? how can I tell for certain?