Usually problems like this occur (when decided the power is not the culprit) when there is a problem with APCI and if these problems persist the best option is just to disable it in the bios. Also, if it does happen again it maybe that XP was becoming unstable. What I would do is right click my computer, and then properties. Then click the advanced tab, then startup/recovery and remove the check from automatically reboot in the system failure section. This way if its a crash, it wont just shut down, it will give you (hopefully) the BSOD and we can then work with the errors listed. Also, going into event viewer would be helpful. Hopefully, though, it is fixed.
If you argue with an idiot he will drag you down to his level and beat you with experience.
I am not a fast writer.
I am not a slow writer.
I am a half-fast writer.