Did I stumble upon a bad mobo?(K8N4E Del)

Hey all, I'm pulling my hair out over my new PC build (my first new build in about 3 yrs). First, here is what I have put together -

AMD 64 3400+
1G PC3200
Asus K8N4E DELUXE
Geforce 6600 GT
WD 80G 7200 HD (swapped this from my old system)
400W Antec PS

So here is what has been happening -

- Great news, able to install XP pretty easily.
- Download all the driver updates for vid card and for XP (SP2, etc.). Install all of the mobo drivers from the asus install disc.
- Everything seems to be working great for at least a few hours.
- Then the pc powers down out of the blue and tries to reboot.
- Reboots fine, so I start to do more stuff.
- Powers down again out of the blue and tries to immediately re-boot.
- NO KEYBOARD DETECTED error message - boot stalls. Weird, wont detect USB.

So I get out my old, non-usb keyboard and try that. Reboots into OS. My mouse does not work (it was plugged into the back USB ports like the orig. keyboard.) I remembered that I have 2 front USB ports on this case so I plug the mouse into those - it works! Plug the orig. keyboard into the front USB and it works now too.

I think I have solved the issue, then another random power-down. This time I get the DISK BOOT ERROR message during POST. I have to put the XP disk back in to attempt a repair install. That crashes. So I try again with a fresh, wipe the drive clean install of XP.

I've been through this same cycle 3 times now, all with the same results. I've finally thrown up the white flag. Does this sound like a bad mobo? Since I'm losing USB support out of the blue that is my thought. But could it also be the PS?

Anyway, I'm looking for whatever guidance you guys can give me. Thanks a ton 8)