Well this is obviously a hardware problem; yet our diagnostics are not turning up any errors. At some point, it's time to stop relying on them and start actually start replacing it.
Given the age of the system and the fact that it's an OEM machine, chances are the PSU is not in the best health. OEMs typically stick a cheap-ish PSU in the system, and as the capacitors age, it begins to provide less and less power.
Usually PSUs go slowly, but something like a power surge may have taken a big chunk of life out of the PSU, so to speak. That would explain why the problems started happening suddenly.
I'd expect that if the RAM was bad, either Prime95, Memtest86, or both, would show some errors. No programs are perfect, but I'd say let RAM be a lower priority than the rest here.
I realize Prime95 supposedly gave the CPU an all-clear, but I am not entirely convinced. That error log I posted in my first post points to a processor problem.
I'd suggest start with the PSU. That may explain why Prime95 found no errors but there was still a processor issue. See if you can borrow a high-quality PSU of at least 500W. If I were to recommend a brand, it would be Corsair, but CoolerMaster and a couple others are good as well. One of the guys on TSF wrote up an excellent article on PSUs, it is definitely worth a read:
Power Supply Information and Selection - Tech Support Forum
If you have to purchase a PSU, check the return policies, just to make sure they will allow you to return it if need be. The local PC shop may be of help to you.
If the PSU is not the problem, the next thing I'd try is the CPU. If there are any other components you can borrow, though, such as RAM and video card, that would give you that much more assurance about what the problem
could be.
I'd expect to see some errors in your event logs pertaining to the hard drive, if a bad hard drive was indeed the trouble. Since I saw none, I expect that is not the trouble; it won't hurt to run a diagnostic. I've found hard drive diagnostics to be the most accurate of the tests; I cannot recall any cases where these programs passed a bad drive. Try running the Short DSTs with
SeaTools.
