Well, I was pointing out the more likely candidates. What you are describing is VERY UNLIKELY to be a software issue. Since it's happening in multiple places, and from two installation CD's, I'm forced to conclude that the likelyhood of a software issue is pretty small.
However, if you'd rather look for software solutions, I'll butt out and let someone else take it.
You have to start narrowing down the possibilities. If you have any spare parts, like other disk drives or memory, I'd try them. Also, try going into the BIOS and setting it to defaults, then check and see if the processor or bus speed is set higher than what it should be. Also, make sure the heatsink is properly attached, and there is thermal bond between it and the processor. I'm guessing it's something about the MB/processor/RAM, but it's hard to say without narrowing it down. The P/S could also be an issue, though IMO it's a bit less likely in this scenario.
I've seen this thing a few years ago, and mine turned out to be a bad MB...
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