Question : Problem: XP computer restarts after cold boot

Recently my small form factor computer (Biostar Ideq 210V) started rebooting after cold starts - it would start up normal and operate normal for about 4 minutes, then automatically reboot. After the reboot, the computer would operate normal until the next time the machine was shut down and started cold again. I checked for heat issues by using a utility and the processor temp never went over 38C. I opened the case, and all fans are spinning normally. The machine is running updated XP Home and AVG Antivirus.
I suspect either a failing power supply or failing memory, but would appreciate confirmation of such, or any other ideas.

Answer : Problem: XP computer restarts after cold boot

did you test the ram?