even when my Dad sends me an email with an attachment I didn;t ask for I don't open it. In fact, many viruses work that way - they come from someone you know because the virus was clever enough to steal all the addrs in your friend's file or your friend sent you and email and didn't know he had a virus that attached itself to the email.

The easiest thing to do is never open attachments ... unless you requested, know what they are and who is sending them. And even then it is wise to scan them with an up-to-date anti-virus scanner.