HELP! Computer shuts down

My home computer seems to shut itself down randomly now without being loaded. I haven't figured out why yet and I'm looking for ideas on a low budget. My friends have told me its probably the power supply, but it had worked for a little while, then while copying some files over it just suddenly started crashing. Some of the things I've noticed is, the CPU fan doesn't seem to spinning fast, the SPDIF optical laser is off, and it shuts down early after Vista is loaded. In the bios, all voltages from the power supply seem to be normal so I'm not sure what's going on. Here's my original build and the new build though I'm reasonably sure it isn't about the new build.

Original build New build
Antec Earthwatt 500W PS Everything the same except
Seagate 250 Gig hd PATA Seagate 300 is dead
Seagate 300 Gig hd PATA MSI 8600GT card (replace 7900GS)
Foxconn AMD 939 nForce 4 SLI MB Netgear Wireless PCI Adapter G
AMD 939 X2 4400 CPU
Hauppauge WinTV PVR 500 MCE TV card
Samsung DVD Burner
EVGA 7900 GS card
2 Gig Ram
Vista Home Premium