At present there is no "supported" antivirus product available for WHS 2011.
Using an unsupported product will cause more problems than it solves. If
you are trying to protect your client pc, you might give avast a try.
I read moneypit as he/she is doing this on the client (Vista), not the server.
WHS 2011 RTM up and running, v.1 gone to meet its maker...