For example, using Microsoft Outlook, create a contact called: @Infected. Set its email address to: PC-has-been-infected@scan-your-PC-right-now.com/
The reason to name the contact "@Infected" is that the @ symbol will sort this contact so it's at the top of the list, but you can use any name you wish. The reason for the extremely long email address is to avoid using an address that actually exists. But again, you can use any name you wish.
When a virus (or other form of malware) scans your address book for contacts to send a copy of itself to, it will grab this first contact and send a copy of itself to the contacts email address. You will then get some sort of undeliverable message that states your email server could not deliver the message to PC-has-been-infected@scan-your-PC-right-now.com.
If you ever receive this undeliverable and you did send an email to this contact manually, you'll know your computer is infected and it is trying to spread itself to your contacts. At that point, you'd better disconnect your PC from the Internet and scan your computer for infections.
About the author: Tim Fenner (MCSE, MCSA: Messaging, Network+ and A+) is a senior systems administrator who oversees a Microsoft Windows, Exchange and Office environment. He is also an independent consultant who specializes in the design, implementation and management of Windows networks.