Basic rule .. .
Don't open/run any file that is sent to you "unsolicited" - even from friends.
If I get a "Hey, just check this out (blah-blah.exe)" email, I immediately email the sender (if someone I know) and ask if he intended to send it to me and if he's virus checked it. Usually I get a response "sorry mate - didn't send it but it looks like I have a virus on my machine" (he shouldn't have opened/run it!!!)
Always check, and double-check, before opening/running attachments
(My mate got an "exe" the other day with the flight paths of the 4 Sept 11th aircraft on a graphic display - haven't checked yet, but he has been told it's a virus!)