Internet security and antivirus for windows 2003 in peer to peer mode

I have a friend who works in a small size organisation and their computers about ten (10) where connected with one server running windows 2003 server on one of the pcs.  My friend asked me to advice him on the type of internet security and antivirus  they can use to protect their systems.  Because the systems are in peer to peer mode, I do not know the type of antivirus he can use please advise me.

What type of internet security and antivirus can they use in a client server mode (peer to peer) system 

Thanks.  I will wait for your advice.