Well, the stuff you've found so far is probably on track - it's likely a corrupt system file or driver. Possibly something that a disk check can fix - if not it could indicate if the HDD is failing. I usually pull the drive and scan on another PC, but if that's not available then a boot CD would probably be needed, as you mentioned, if the recovery console is not installed.
While it usually results in a bluescreen/reboot, sometimes what I've seen is that XP installs on SATA hardware were done in ATA compatibility mode, and if the BIOS resets to AHCI mode, the system will crash during startup.
The standard reseating RAM, trying DIMMs one at a time, etc if applicable wouldn't hurt either, if you haven't already done so.
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