I was using my computer and it shut off and now it wont start no fans or hard drive spin up or anything I need to know if this is a motherboard or Power Supply problem

about a month ago it started to lock up and I had to reboot it every time and sometimes it would just reboot. This usually happens when I was playing games or running alot of programs at the same time

here is my computer
FSP power supply 480W 500w Peak
XP Pro
AMD X2 4200+ Dual Core CPU
ASUS A8N-SLI Premium
2 GB OCZ Ram
EVGA Geforce 7800 GTX 256 MB
Sound Blaster Audigy 4
Plextor DVDR PX-716A
Maxtor 6L300SO 300GB SATA
Western Digital WD3200KS-00PFB0 320GB SATA 3.0
Viewsonic VX922 19" LCD Monitor
Bose 2.1 Speakers
Thermaltake Armor Series VA8000BWS Black Aluminum/Steel ATX Case