> ...then something is wrong with the WinXP install.
>
> The most radical approach would be to reinstall XP, which *should* keep some
> user settings but not all. So last resort only.
>
> If you can get to Windows, you can use msconfig.exe to disable startup services.
>
> I guess you could also try to fudge some of the INI files (win.ini, system.ini);
> but that's pretty dangerous as you risk breaking more thigns if you don't back
> them up.
>
> It could maybe be the hardware too - you can try using the memtest liveCD (boot
> in Linux by itself, then scans the RAM for physical issues). If the machine has
> extension cards, you can remove them to see if it prevents the BSOD.
>

Could be a dodgy memory module or CPU fan. Try booting into a LiveCD of Linux and playing around for a bit to see whether it stays on.

Hardware aside, there's also the option of re-installing Windows over the top of the existing installation. It might break a couple of programs, but should mostly work.