Your question would be better asked here:
http://forums.microsoft.com/windowshomeserver/default.aspx?siteid=50
However, as a long time Windows Home Server user, I will offer my opinion. ;-)
Since the WHS box is not used to access the Internet or run programs, you really don't need virus protection installed, in my opinion. Files written to the server or read from the server will be scanned by your local virus protection. If your WHS web site and folder shares are open for others to upload to from the Internet, if an infected file is loaded to the server, it is harmless until someone tries to execute the file - once again, local protection should step in.
You can also point your local scanner to make a manual scan of the Shares on the WHS box on occasion,
-steve
