Computer Help

My computer has been crashing and restarting when I am playing games and sometimes even when I am looking for a file on my hard drive

It eventually wouldn't even load windows, it would get to the windows loading screen and then shut off, and then it didn't even turn on at all

I am thinking that it is a Power supply problem but it also might be a video card issue too

I took everything but the basics out and without the video card the fans and HDDs spun up and I got the no video bios beep code thing and but when I put the video card back in it didn't start at all. when I pushed the power button the lights on the fans flashed for a second and then it did nothing

here is my computer

XP Pro
FSP Group (Fortron Source) AX500-A 500W
AMD X2 4200+ Dual Core CPU
ASUS A8N-SLI Premium heat pipe version
2 GB OCZ Ram
EVGA Geforce 7800 GTX 256 MB
Sound Blaster Audigy 4
Plextor DVDR PX-716A
Maxtor 6L300SO 300GB SATA
(x2)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