i re-installed XP pro a few days ago because of a problem with the new nvidia graphics drivers, which caused it to fail at boot (and in safe mode). i now seem to be having a similar problem, but my system boots about every 3rd time i try it. What could be causing this irregular behaviour?

Are you still using the same card and drivers?

What exactly happens when it fails to boot?

Does it pass BIOS POST? Does it crash in XP?

Still using same cards, but i downgraded to my old nvidia display drivers because i know they work properly. It passes bios every time, but windows crashes at various different points in the progress bar screen. A bluescreen error appears, but it shuts down immediately, and even when i go to advanced startup options and select disable reboot on failure, it still reboots.

Try booting your XP CD into the Recovery Console.

I would first try chkdsk /r

And if that doesn't help try fixboot

Description of the Windows XP Recovery Console

ok, thanks