There are only a few software packages available for WHS specifically. None of them are free as far as I know. Foolish of Microsoft to not offer Security Essentials for the platform, but foolish is nothing new for them I guess.
Personally, I don't think it's a huge deal. You're not going to be using it for web browsing or anything and all the files that get onto the server from your clients should have been scanned by the AV on the client machines.
I decided to live dangerously and haven't installed an AV on my WHS. Been over 2 years now without any issues. I do run a malwarebytes scan every couple months to double check though. I couldn't see spending ~$50/year for AV on a glorified NAS...