Some better pranks:
Take a screenshot of his desktop (maybe with some windows, pr0n websites, or the start menu open), and then set it as his background. Remove all the desktop icons, hide the taskbar (resize it to height = 0) and voila, permanently "frozen" computer.
Put a shortcut to shutdown.exe in the Startup folder.
There was an old application that ran in the background and made farting noises every few minutes -- drop that in the startup folder.
I'll post more if I can think of them...
It's more fun to link about 50 songs files together into one song and have it play them all, with no easy way to stop it, when the computer it turned on.
IIRC Windows only plays the first 15 seconds of any sound file on startup.