HELP! Constantly rebooting during XP logo splashscreen

I have windows XP on a hp pavillion desktop. Within a couple seconds after the windows XP bootup splashscreen is displayed, the computer reboots.

I am unable to boot into safe mode, or "last known configuration that worked". When I tried the "disable automatic restart on system failure" option, I get an UNMOUNTABLE_BOOT_VOLUME error on a BSOD, so I googled on that and tried booting off the XP install cd into the recovery console, and trying chkdsk, fixboot, and fixmbr to rebuild the master boot record, but to no avail.

What can I try next?