Not to argue Nihil, but I just had a system that was had run fine for over a year and then started giving STOP 0As at boot. Wouldnt get in with safe mode, wouldnt reinstall, etc. Turned out it was a bad dimm. The memory tester in bios isn't the best, to really check a stick of memory you need to run a good utility against it. I've also seen this with a bad bios chip on the board. Particually a K7N2 Mb with an athlon 2400, system got hosed, would post, detect all the hardware, but did the fun stop messages at boot, and funnily enough, if you went into the bios setup, and just let it sit long enough, it would lock up in bios setup too. Seemed to lock or generate a stop after about 2 minutes of being up.

Trying to boot into redhat is a good suggestion, if he boots into it and it has a kernel dump or whatnot, that might help pinpoint the problemo.