Virus Protection of a NT Server

I set up a NT4 server with IIS4, DNS, and Exchange server 5.5 on the same machine. This NT4 server machine is used as the front end to serve multiple web sites of a small company. The NT4 service pack 6a with roll-up security patch was installed on this server. This server is directly connected to a router to get on the internet. Of course, this server is also connected to the primary domain controller through LAN.

Questions:
(A) I would like to know what virus protection software thatI should install in order to provide sufficient amount of virus protection for this server.

(B) Is there any other software that I should intall to prevent “the intruder”?