Computer shuts down then freezes

Over the last few weeks there have been assorted problems with my friends PC. sys files going missing, BSOD, DLL file probs. Tried reformatting drives and reloading Win XP pro without making much difference, if anything faults became progressively more frequent and severe. Noticed via Asus probe a low voltage >3v on the 3.3v rail. Got new case with 460W PSU. PC ran ok for 4/5 days them just started switching off to reboot after a few minutes only it does not come back on it seems that no signal is being sent to the monitor although fans, CDroms and HDD are spinning up. Tried 2 graphics cards ATI radeon 9800 pro and asus FX5200 symptoms are the same. Tried 2 different monitors same result. If you leave the PC a while with the power off it boots up into windows for a few minutes and then goes into same routine. Ran HDD diagnostics, all clear and Microsoft memtest shows no faults thinking it maybe Mobo or CPU related any advice would be welcome. System spec

Athlon 2000+
Asus A7V333 Mobo with Via 4in1 drivers
1Ghz 2700 DDR ram
ATI Radeon 9800 pro
2 HDD 1x80Gb, 1x40Gb
1DVDRom, 1CDRom
460W PSU came with case