Choosing and Anti Virus program

I manage a network with 70 computers in one site and about 60 others in diverse sites soon to be connected by WAN links.
Internet access is provided through a server with MS Proxy 2.0 and a DIALUP connection. The NT 4 DC also has Exchange 5.5 on it.
Presently a few users (5) access POP3 accounts through the winsock proxy.

My question what antivirus program would best protection under this situation?