Try a WindowsXP repair operation. I think you need to boot up with the WinXP discs, which should present you with a repair option.
Also, if you have the recovery console installed, try doing some HD integrity tests (chkdsk /p). I've linked WinXP instabilities to poor HD integrity in the past (back in the days when I was running my system of an IBM DeathStart HD).
You may also want to remove all other hardware components, and disable all BIOS peripherals. Your BIOS may have an option to revert to a fail-safe configuration, in which case, use it. Also, are you over clocking anything? Reset everything to factory settings and slowly re-build your system one component at a time.
Good luck.
- Mike
