The last BSOD I got was 0x0000007E; before that it was the one that was driver_irql_not_less_or_equal (I don't remember the number code). Everytime it blue screens, it's something different, which is why it's such a puzzle for me.
When it's random like that it usually means something low level and major is starting to fail. RAM (rare but possible), CPU (overheating? check for dust), power supply (again rare), or the hard drive is starting to croak (the most likely candidate).
It can mean some drivers have gotten out of whack with each other, but that usually generates the same error over and over. However, if you've updated to SP3 then you've got the latest and greatest of all the bits and a software conflict is getting less likely. Maybe it won't crash again now that you have...fingers crossed.
Either way, if the computer is old enough to be running XP by default, it's old enough to be reaching planned obsolescence.
Make sure your data is backed up to a safe place, and save your pennies for something new, because your machine is showing all the signs of old age.