Bizzarre boot issue...

[Siorus]

I've got a pair of HP Elite HPE204Fs that I'm trying to load XP-64 on. Normally I build all of the computers in the office myself-or have someone local build them-but I needed something really fast, and these had Win7 Pro on them (which we'll be upgrading to later this year), so... I went with them.

Anyhow; we're not ready to move to Windows 7 yet, and I had a couple of spare XP-64 licenses sitting around, so I'm putting it on these machines. I installed it on one machine, and it is now doing something that I have never seen before:

If I don't have the Windows CD in the DVD drive, the machine won't boot. It POSTs, and then before reaching the Windows boot screen, it restarts.
EDIT: I suspected a driver issue, but I just formatted and did a clean install. With no drivers loaded, it still does it.

I don't think it matters any, but I did replace the hard drives in the machines with WD 500GB Caviar Blues (so I can just slap the 7200.12s they came with back in when we go to 7) and set the SATA controllers to IDE mode instead of RAID-which is what HP had it set to. The Caviar Blues have both been through a full surface scan and a zero fill, so I doubt the drives are defective.

EDIT: I've tried changing the boot order to boot from the HDD first, that doesn't do anything.

 

[Siorus]

Just to add to the bizzarreness:
Only one of the two machines does this. The only difference between the two machines is that the one that's exhibiting this issue has had 3 windows installs on it, the one that's not has only had 1:
-1st install: Clean drive, freshly zero filled. Installed all of the drivers for the machine with the CD in the drive, rebooted several times. Removed the CD, rebooted and now it won't boot.
-2nd install: Repair install; no change
-3rd install: Quick format + clean install; still doing it

I'm going to do a full format and try again, but a quick format should be sufficient. I'm starting to suspect a bad motherboard. Anyone have any other ideas?