If your virus checker is kept current it will still only detect KNOWN viri.... However, you are correct, if it happens that a new virus is of similar design to an allready existing one then the AV will still match signitures and report it as "POSSIBLE NEW MALWARE"
This does not keep you safe though as code can vary meaning it is possible to evade all AV signitures..
As you said, it is better to be wary of attachments from people you do not know.
"hmmm - why would an up-to-date virus checker not detect it? Are you saying that they've finally written a virus that cannot be stopped?"
A little naive i'm afraid, sorry if I sounded patronising however, a glass of wine to many last night me thinks
