What to adjust in the Windows XP Registry to play tricks on classmates

Ok so im going for My A+ and Net+ and currently in the software version of A+ and we will have the opportunity to screw around with a computer and mess it up(nothing off limits just as long as nothing is fried, we can easily load windows again) so there has been quite a bit already done to each other and im wondering if anyone has ideas of what we can do, the registry seems to be a popular place for us to go, im thinking just for fun I'll create a batch to run on startup the will format the C: drive lol but it wont take long for them to figure out what happened, so please post your opinions and just in case im not sure how to do what you say please post instructions as well.

Thanks,
Ryan