Today with most PC's connected to the Internet, it is very easy to become "infected" by a virus, trojan or even spy-ware. One the infection has been absorbed by the computer it often starts a "Time Bomb" ticking away inside. It may be minutes or many weeks before the infection "starts up" but once it does, depending on the type, it may cause damage to the hard disk, or begin to steal addresses from your email program and use them to send Emails to the people in your address book.

What is important, is to "remove" the infection, and then to stop it re-occuring, installing a good Anti Virus program which will check the machine on a regular basis.