It sounds to me like it might have something to do with your machine's temperature. You can check to see what the temp is set to in the BIOS when your computer is booting up. You should be able to find out what yours is set at and change it to a higher setting. Before doing that, though, you might want to find out a "safe" temp to put it at.
Do you hear any sounds just before it reboots? I had an older computer that would give off an annoying buzzing sound and it would reboot. I had to go in and raise the temp limit.
It could also be that the power supply is getting too warm too fast. Being it's a built system, it may not have a strong enough power supply and you may need something stronger. I would check the temp first though.
Recently I had a computer built. I installed Windows XP Pro and the computer reboots constantly. Not sure what to do to get it to stop doing this. HEre is the hardware I am running on the machine:
Intel Celeron D 351 3.2Ghz LGA775 System
ECS P4M800PRO-M LGA775 1066FSB Dual Core
Intel Celeron D 356 3.2Ghz LGA775 533FSB
STANDARD COOLING FAN & HEATSINK COMBO
512MB PC2 4200 533Mhz DDR-2 Dual Channel
80GB SATA150 7200RPM HARD DRIVE
Dual Fan Combo Cooler For One Hard Drive
VIA UNICHROME PRO 3D ON-BOARD VIDEO CARD ADAPTER
CD/R-RW - INERNAL CD BURNER DRIVE IDE
REALTEK ALC655 5.1 CHANNEL SURROUND SOUND AUDIO ADAPTER
VIA TAHOE VT6103L 10/100Mbps ETHERNET ADAPTER (LAN)
ATX Black MidTower Case W/ 400Watt PS
Standard Power Supply With Case Selected
Case Cooling Fan 80mm (Regular Speed)
I'm wondering if I need to get better memory? Anyone have any suggestions of what it maybe and what I can do to get it to stop rebooting? Sometimes it reboots when I'm not doing anything. Other times I will just be using Internet Explorer or running Windows Update and it reboots before the updates can finish. I've also had it reboot in the middle of installing anti-virus software and firewall softwoare. .