Just to explain what I meant by why to Sophocleese, even a friend can unwittingly send a virus so if an unexpected attachment came, I would still be wary.
The only one that has arrived to me in this way is a little beastie called Happy99.Exe. This one is not really a threat to PCs and is quite well known. Basically it attaches itself to emails and just causes a lot of unwanted junk on the web. I am not sure but believe that it has been know to reach epidemic levels in some newsgroups.