I recently got an email with the following text:

If you get such an email DO NOT open the attachment, just delete it. The attachment is a virus. (It's a good idea to not open any attachments, especially emailed from people you don't know.)

Or even from people you do know, unless they've previously told you they're sending you something. Many viruses attack email address books, so it's not alarming to receive a message with attachment from a friend.

So, unless you're expecting it, don't open it!

And keep those anti-virus definitions up to date. Most anti-virus vendors will have an update to catch a new virus within a matter of hours. But if you don't have that update, it doesn't help you.