The first thing I would look at is:
did you install any new hardware or software prior to this happening?

The next thing would be a hardware problem...and particularly the memory.
Download memtest and run it over night on your system. If it shows any errors at all...replace the ram stick that is failing!

Memtest86 - Download Page

The other issues is to open the computer and systematically remove and reinstall every connection, plug/socket, card that you have in the computer in hopes of it being caused by a bad connection! In doing this you should remove the add-on cards seperately and try to boot the computer....if it is a bad driver file then this will allow the system to not load the corrupted driver if the cards(hardware) is not installed.

Another issue would be to remove the drive and slave it to another computer and navigate to the windows properties and deselect the auto reboot tick so that you can reinstall the drive and have it boot to show the error code to give a clue as to why it is happening!

The last choice would be to just do a system repair using the second repair option on the XP installation disc to restore the operating sytem