Does it take a set amount of time between getting the errer? Then it is a bug
Does it always happen by a schedule, like at 1 aam, 5 pm, ect, ect? If so, you have some software on there that will shut down the computer at a set time.
Does it always happen when the computer isn't in use, after x minutes? then you computer is set to shut off after x minutes. Don't know how to fix this one, though, as I don't have XP. (Bet it will probably be in power settings or something) Or it, too, could also be a program.