If you aren't expecting an attachment, don't open it. If you have any reason at all to question an attachment, e-mail the sender and ask about the nature of the attachment.
If at all possible, avoid sending e-mail attachments. You can paste the text and sometimes the formatting of most documents into the text of e-mail messages, and they work just fine without the risk of carrying a virus.
If you're on the Internet or if you share computer disks with anyone, keep your virus checker up-to-date. Have it set on "auto-protect" (or whatever your constant virus monitor is called). Also, scan your hard drive for viruses once a week.
Most often, you get virsuses from people you know, people who don't even know they have a virus. This is true in life, as well as in computing...
-Joe Offer-