Anti-Virus Software

Please don’t laugh. In a small NT 4 network (>15 client computers, 1 iMac, 1 PDC, 1 BDC, 1 stand alone server which are connected through a NAT’d router to a SDSL line) what would be a good candidate for an anti-virus software? Also, (and this is the part I am asking you not to laugh) since only the clients are accessing the internet why should we be concerned with server anti-virus software?