Another good trick is to set windows to play a recording of something really messed up whenever e-mail arrives. It's under control panel, sounds and audio device properties, New Mail notification. Because e-mail, while connected in the office, is totally random, they will never correlate the incoming mail with the sounds played and have a REAL hard time figuring it out. Just find or make a wav file, copy it to the computer, and change it to play the wave whenver email arrives.
Yes it happened to me. Been doing IT for 10 years and I couldn't figure it out. My PC was randomly playing the Scooby Doo theme song. Since it was based on incoming e-mail, it would just spit it out every 10 minutes or so as e-mail from someone would arrive. I and most people keep the e-mail application open all the time since it's a business standard method of communication.
Since it wasn't a real virus, NOTHING I used could find one. Was doing searches on "scooby doo viruses and such without luck , all the while the damn computer would suddenly start playing scooby, dooby, doo, where are you.." (there isn't any way to stop the entire wav file from playing either once it starts as it is a "system sound" when configured this way) Since the mail program is kept in the background, I couldn't correlate incoming e-mail to the song. I was REALLY pissed and about to reformat and re-install when my IT coworker friend confessed (I was wondering what all the snickering was about in the background)!!