Been there, done that...
There's a dwindling number of single license AntiMalware products that will run on Windows Server OS. It would be great to be able to use use 'avast! Home' but Alwil choose, like amost all AM providers, to limit compatibility of Home-user products to Home-user OSs.
Shortest distance between problem and solution? I now have a few HP and IBM Server 2003 file server machines running
Rising 2009 at what is for a commercial user, the ridiculously low price of USD19.95 each, but have not tried it on Server 2008.
Panda's Free Cloud Antivirus might work. I'd ask on the forum first if I were you.
I don't think MS's own
MSE will work but I haven't tried it either.