Hahah,
I think every experienced computer user can relate to these things,
Some things I have done:
I took a screenshots of my mates word document then set it as his wallpaper
I have put all his documents in a folder and hidden the folder
I have edited the clippy thing to say really rude and insulting things
I have used a fake virus to trick our network administrator, he freaked out and nearly reformatted the whole network haha
I have put a moustrap on the leaver of my mates desk chair (one of my eviler ones)
I changed our network administrators background to a half naked guy (admin is male) - Was especially funny when he had to do a presentation on projector in front of whole school
Turned the monitor of one of my computer illiterate mates upside down
I edited a picture of a mates mum to show a lesbian pic (and set it as his background)
Uhm, I think thats about it. Will post if I think of some more,
Take care everyone.
cheers,
Korrupt