Go to the guy's computer and make a copy of an often used program. Move the original to a safe hiding spot(c:\windows\system works great). Rename the copy's extension( the ".exe") to .txt. Open it. Find the part that says "This program must be run under Win32" or "This program can't be run in DOS mode" and change it to "Your system has been INFECTED!!!!! Happy virus checking(It'll never clean or quarrantine me)!" Then add some characters to the end. Now rename it's extension to .exe. It should now look the same as the original.
