It could be. First send him an email (anonymously) with something like 'Great Poker Tornament Players Required, Your Invite' then get him to download some stupid little app (if he doesn't know much about computers then com.exe might be enough). Then, later that day, change his permissions so he can't install and also change his password so he can not enter his account.
When he comes to you to ask you what is going on you ask him if he has downloaded anything recently.....
Irony. My virus checking was scanning my computer whilst I was typing this and it found a virus.

That's a pretty good idea. I think I may try that. The only problem is that I don't know what his email is.
I found a few small prank apps that mess with the mouse pointer. I'm going to log into his account and stick it in his startup menu (rename it to something that sounds official). I'm just curious to know if this will affect my startup though. If so, all I have to do is hit esc and I'm back to normal.