New GA-7VT600 = No boot

Hello all. Bought a new GA-7VT600 mobo for my old XP1700+, 2x256MB PC2100 RAM, Antec 350w PSU, GeForce2 MX400, DVD, 80GB HDD, SBLive 5.1.

Put the whole system together and tried booting into old WinXP install, and it kept restarting. I tried safe mode and saw mup.sys was the last file loaded before automatic reboot. Tried disabling mup.sys in system recovery console only to have it load sisagp.sys before reboot. Tried disabling this too, and same thing happened with ndis. So it's not any particular driver.

I've been reading stuff on the Internet about what at first appears to be a mup.sys error, and I've tried everything: changing every possible combination of BIOS options (including disabling every onboard feature), taking out first one RAM chip, then the other, changing which slot the RAM is in, changing video cards (I tried a GeForce3 and a Radeon 9200 in addition to the GeForce2), moving all but essential hardware.

Many of the posts I read indicated wiping HDD and reinstalling OS did nothing to rectify the problem, so I haven't tried that (I'd also rather not lose my data). Also note that the only new pieces of hardware are the mobo (upgraded from a K7S5A) and the PSU (upgraded from a 200w Compaq), though I did unplug everything else (I went to a new case).

Any musings would be appreciated.