Windows XP OS restarting randomly
I recently bought a new eMachines PC (what that a mistake?). I've had it for a month and have run into several problems. First, the motherboard went bad 1 day after the return policy expired, and since I upgraded it myself I voided the manufacturer's warranty. Anyway, Since the motherboard has been replaced Windows XP will randomly reboot. Trying to use the error report (is anything more useless?) I got back certain messages saying that there is an unknown device driver on my PC. I've gone into the device manager and nothing is showing as unknown. It has also said that there is a video driver problem. I have a GeForce 5500 card with all of the drivers, so I don't know why that would be a problem. The only other thing the error report will tell me is that "your OS has recoverd from a serious error". I've gotten several virus and protection programs. McAfee came pre-installed on my PC and it seems to operate OK, but whenever I try to perform a virus scan, my system will reboot midway through! I also have Registry Mechanic that will detect problems and repair them, but it doesn't resolve the restarting. I've used Ad-Aware SE and I haven't found anything there either. At times I'll get kicked out of programs I'm running.
What I've got is a AMD Sempron 3400+ (2.0ghz) processor, 1 GB RAM, nVidia GeForce 5500 graphics chip, and Soundblaster Audigy sound card, and 160 GB HDD.
Lastly, the only other thing I can think of is when I originally bought my upgrades, the main board had only 2 PCI slots in it. In order to install the graphics chip and sound card, I removed the phone modem card. I have a new board now w/o the modem card, but could that have done something to the OS?
What's wrong with my PC?!!