Some of the best computer pranks I've seen were:

Changing all the aliases or shortcuts on the desktop for files. Click on MS Word and you get Filemaker, etc.

Person opens their email program, triggers a script that writes an email to the boss saying person quits, going to work the drive up window at Burger King.

Probably the best pranks I've participated in:

Put a tape recorder on top shelf of bookcase at library, minute of silence to allow me to get back to my table and then there's all sorts of screaming. Librarian runs back to that part of the library, sees nothing.

Guy passed out during the Super Bowl, had $200 on the Chicago Bears to win. When he woke up in the fraternity house, everyone told him the New England Patriots came back to win soundly. Had him going until he went out that afternoon for classes and saw the newspaper.