Oh for crying out loud. It's doing its normal housekeeping chores on the system and writting all of it to logs. When you the user are not banging away on the system and it's been idle for X amount of time the XP OS takes advantage of the idle time to do background chores.
Don't format your system. It will do the same thing again. You can turn off the event logs and that will stop some of it but it's not a real smart thing to do. My P4 looks like a disco light when it's busy at night. But after a while when it's all done it spins down the drive and stops.
Oh and if it is a bug or nastie, when not online go in the network settings and disable the network device.