A few weeks back my power supply exploded (smoke coming out the back, burned plastic smell etc). I finally got some extra cash saved up to get a new one. Installed it last night, and it would turn on, but it would come up to the bios post screen saying bios post had failed and to use the default configuration. It also dies about 1 minute after powering up, regardless of what its doing. The less time between power ups the quicker it dies. I tried unhooking all the components until it was just the motherboard and CPU, and it still did that. I got into the BIOS and reset it to default values, and that didn't work.
Then I read about resetting the CMOS and figured I'd give it a try. Well, I finally figured out how to do it on my mobo (ASUS P5N-D) and low and behold it worked. It gave me a checksum error saying it couldn't load the bios settings and that it was going to use the defaults. I gave it the go ahead and it booted up just fine. However, when I shut it down and tried it again it went back to the old dies after a minute fail to post problem. Resetting the CMOS again fixed it, but that's a horribly inconvenient way to boot up your computer.