hanging in windows XP - WARNING! tech nightmare ahead!

Yesterday, after a wonderful game of Countestrike Source, I thought I'd try out the new Ubuntu Linux beta. Why not? I have the HD space.... So I rebooted into the install CD, resized my secondary drive to have space for Linux partitions, and went about my installing business. Unfortunately, the installer crashed- it couldn't load the Linux kernel. Apparently other people have had this issue with my motherboard (doh!). Oh well, I figured - I'll wait for the actual release. No harm done.

So I booted back into XP, and everything seemed to be going just fine, for about 60 seconds. Then BAM! The system froze, just as I got into CSS. Reboot again - this time, to make sure everything is all right with my hardware. BAM! Another freeze, just poking around in Device Manager!

So: the problem is a consistent hang. BUT ONLY in Windows XP. Here are the various tests I've run, in no particular order:

- check component temperatures/manually set all fans to max/add a room fan pointing into the case - no difference

- underclock the CPU, up the voltage on RAM, CPU and FSB - no difference

- try with only one stick of RAM (tried different sticks and different slots too) - no difference

- Replace the video card (changed bus too, from PCIe to PCI) - no difference

- change the hard drive (and bus from SATA to ATA) - no difference

- reinstalled Windows - installer runs fine, but once the system is up and running... BAM!

- booted Ultimate Boot CD:
- individually tested both hard drives with manufacturer utilities
- ran memtest for over 6 hours without errors
- ran CPU burn-in for 5 minutes without errors
- presently running Merseine Prime test (I'm on test 9, so far no errors)

To my mind, the only component left is the motherboard. But why would the motherboard have a hard time ONLY in windows? Heavy load to the CPU/RAM seems to be no problem, so long as the graphics mode is VGA. But the video card is fine - I still get freezing when I replace it! Maybe it's the PCI bus on the mainboard? So I get a crash when too much info is passed over PCI? And why would this be triggered by a partial Ubuntu install? Or is that coincidental?

Anyone care to lend an opinion on my diagnosis? Better still, anyone have any ideas on a test I could do to confirm my diagnosis?

Thanks!

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Intel Core 2 duo e6400
Gigabyte ga-965p-ds3
2gb Wintec RAM in 512MB sticks
WD 80gb SATA HD
Seagate 80gb ATA HD
ATI X1950XTX 256mb video card
Hiper 580W PSU

yeah, I ran the partition checks built into the UBCD, and even rewrote my MBRs and MFT records. No dice.

BUT I did solve the problem. I figured that whatever the partial Ubuntu install did, it was supposed to be only temporary for the duration of the install. So I bludgeoned my way through a complete Ubuntu install - no easy task with an ICH8 chipset! I made sure everything worked in Ubuntu (which means that now I run Beryl .... hot!) and then booted back into WXP. Magically, everything works now!

Thanks for your help, I hope this can help someone else stuck in my position!