...or sometimes before the monitor even has a chance to warm up. I was running an Intel 3.0E prescott on a piece-of-crap PT800CE-A motherboard with a 6800GT and 1024 MB of PC3200 ram. The other components were a 52x CD-RW, 60 Gig Maxtor SATA hard-drive, a floppy drive, and two PCI cards.

Yesterday I spent quite a bit of time getting a fresh copy of XP Pro to install on my new SATA drive. After six hours, I got it working. But today, the ASUS P4P800-E Deluxe motherboard I had ordered came in, and I installed it. Boot up, ran for about ten seconds in POST--checked RAM and got to the area where it should recognize the SATA drive--shut down.

Power supply? Went out and purchased a new 500-watt power supply. Plugged it in, same symptoms. Except sometimes it doesn't even get to POST. So what do you think is wrong with the computer?

Also...often times before the computer shuts down, there's a strange but subtle high-pitched squeal that's not very noticeable unless you're close to the computer.

I've just rebuilt a Pentium III system. Spent a good few hours installing Windows and a few applications and all the Microsoft security updates. Just had a couple of things to finish it off last night before shipping it out to the customer today - install the Epson printer (this was a bit of an unknown as I never use Epson) and chuck in the new graphics card and install the drivers. Did both and it wouldn't boot after that - kept restarting during the boot sequence. Formatted and spent another few hours doing it all over again. Same problem. AAAARRGGHH !!

Turns out that the new 64Mb graphics card pushed the power supply over the edge. It was just too much to deal with along with quite a few other bits inside the PC. If you've got a 500w PSU, that should be more than enough power but you still might find that there's something wrong with the graphics card. If you can get your hands on a smaller graphics card to test it out, it would be good to rule that out even if it's not the problem.

To get through the POST, a PC need a processor, RAM and graphics.

Alternatively, try resetting the BIOS by unplugging the system from the mains and disconnecting the CR2032 coin-type battery from the motherboard for a couple of minutes. This will set it back to factory defaults and might undo any incorrect BIOS settings that are confusing it.

Also, try unplugging all of your drives and see how far it gets then. If it gets to the point where it tells you it can't find a boot device but it doesn't shut itself down, then you've identified a fault in the drive configuration.