There are a couple of AV solutions for Windows Home Server. Avast! has a version specifically for WHS; it integrates with the console and allows controlling Avast on up to 10 home computers that are joined to your server. Fsecure has a version as well, as does BitDefender. McAfee has a product for Windows Home Server as well, but I believe that's only distributed with certain OEM servers (HP, Acer) and isn't available for an end user to purchase.
I'm not on the WHS team, I just post a lot. :)
Ken, FYI, BitDefender does not have a WHS-specific version and in order to install it, it requires logging into the server desktop (you know what that means... :) ), plus a
patch just to get it to install at all.