The old Avast does not work on WHS 2011, or at least mine does not. Unfortunately I renewed my license for 4.8 earlier this year for WHS version 1, and had the EX470 motherboard die about a month ago. I was holding off going to WHS 2011 until I could get virus protection for it. Long story short, I built a new server using WHS 2011, and no virus protection the same weekend the 470 died.
Now I have kids, and on the old WHS it would catch files being uploaded via a web interface. One of my kids did infect the WHS 2011 server with a file from a friends house when he uploaded it remotely using a web interface due to no protection. I have in the past sent emails to Avast Customer Service trying to find out when some sort of protection would be available, but no response from 3 of them, and the only response I did receive was to purchase a regular server license and see if that would work for my needs, and there was no word if Avast would actually release a WHS 2011 version. I have shut off remote access to keep it from happening again, but beware if you are allowing folks to add files remotely, and from any system that is unprotected.