Never *assume* your virus protection is good
I used to have a flight instructor whose favorite expression was "You *assume*! You *assume*! Son, you know what *assume* means.....", but you did not come here to hear about tales of me learning to fly a helicopter. Anywho, besides landing on the tops of mountains, you should also never assume that your virus protection is good.
There are a couple of nasty viruses going around right including variants of Blackmal. Organizations that have "pretty good" virus protection strategies are getting hit. Ensure that EVERY workstation on your network has virus software installed and that the signtures are being updated at least once per day. Make sure that your mail servers have software scanning all inbound (and outbound) mail and that the signatures and scanning engines are up to date.