comp freezes, now won't post

My roughly 2yr old system has been running great but lately its been freezing up (at idle, or when just browsing web for instance), and now it refuses to post.

I'm still trying to figure out if its a hardware of software problem.

I dual-boot XP-x64 and XP pro(x86) that I have on 2 separate drives. At first the problem started on my main (x64) boot, but then it was happening on both OSs' (if I caught a virus on one it shouldn't spread to the other right?)

The freeze intervals kept getting shorter and now I cant even get on it (it powers up, fans start spinning but no beeps and no POST).

I disconnected all peripheral hardware, tried both ram sticks individually in all slot combinations, the cpu heat sink always felt cool to the touch.

I'm not sure what this problem is typical of, maybe someone can help me out.

Thanks,
Biostar nForce550 AM2; A64-X2 4200; (2x1gb) corsair XMS2 6400; Radeon x1800xt; Antec 500w psu; 2 HDs; dvd-rw.

^followed advice, still nada

I disconnected both hard drives, still no POST.

I'm beginning to suspect the vid card, it seems that stuff is going on I'm just not getting any picture. I'll swap in an old pci card I got laying around.

In the mean-time any other ideas?