Computer won’t REboot

The PC, [Mobo Nvidia nForce 2 (AN35S00R) with 512mb (Dual DDR act.), AMD Athlon XP 2800, and 3 drives (2 Quantium 30gbs and 1 Western Digital 100gb)] will not reboot. After every update, change, new app. load, the PC requires a Cold Reboot. Soft boot leaves you stuck at the POST screen, where the machine stalls out…Any ideas? (WindowsXP Hm SP2 is the OS)