It's been a while since I was running XP beta 2 but I know by default that instead of showing an error, the program that caused it, registers, and a stack dump on a pretty blue screen... it just reboots. It may log the incident by default but I don't remember exactly where.

Either in the "My Computer" properties, or start ->run -> MSConfig (I think) you will see option somewhere to change how BSOD's are reported. You can tell it not to reboot so you can look at the information, and tell it to use one of three reporting methods, one I think is 64K dump, one is active memory dump, and the other is total RAM dump. (for when your data is THAT important... not recommended)

In any case, check out your event log directly after a reboot and see if it logged the error. To say it crashed with no other clues but "Running XP and it's crashing while I'm out"makes things difficult, especially if devs or testers don't have XP themselves :P