I recently bought an Intel Celeron 2-gigahertz processor as well as a new mainboard (with built-in video card) and a new power-supply (500 watts). The computer consistently locks up after roughly 15-30 seconds from being powered up. This happens whether I let the system start loading Windows, or if I spend time in the BIOS screen; no matter what I do, the computer freezes after running for maybe a half-minute. Sometimes I get far enough along for Windows to begin detecting-and-installing the new hardware; other times the computer locks up more quickly. I've tried various different BIOS settings, but the computer still freezes up if I'm in the BIOS screen for more than 15-30 seconds.
I am using my old RAM (192 megs in 2 DIMMS from a Gateway 400-megahertz system). The documentation for my mainboard says it will support DIMMS, provided I have the proper jumper-settings (which I'm 99% certain I've done). Also, the built-in video card uses this RAM, and my mainboard documentation suggests I wouldn't get any video if the RAM weren't working. I have no video problems ... just the system hangs.
I also have a CD-ROM, floppy drive, and three hard-drives (40-gig, 30-gig, and 10-gig) plugged into the system -- none of which are new. Because of the constant system freezes, Windows now routinely begins scanning for "improper shutdown" errors on my hard-drives ... and subsequently locks-up again during this process.