office pranks

[FileNotFound]

A MUCH nastier thing to do would be to mess with his hosts file.

If he's got a favourite website that he seems to visit daily (Like maybe http://grouphug.us or www.cnn.com or whatever), you can make his browser go somewhere else every time he tries to visit one of those sites...like maybe something along the lines of The Worlds Most Annoying Webpage ( http://www.mostannoyingwebpage.com) or better yet tubgirl.com or goatse.cx...(Not linked for a reason)
To do this you need to resovle the IPs of these addresses and then enter them at the bottom of the hosts file in the format of:

IP Hostname

as in
123.123.123.123 www.cnn.com

All traffic to www.cnn.com will go to 123.123.123.123, where 123.... is whatever you want it to be.

The benifit of this is that the boss will probably have NO idea how to fix this and there is a good chance that the IT staff will also be totaly clueless. Better yet, if it's a site that he has no business going to at work in the first place, chances are that he won't tell anyone and just be misserable.