Even though SBS is based on the same core OS as XP 3/4s of software out there that would run on it will not simply because it's labeled as a Server OS. As I'm sure you've found out as well, software for servers is also much more expensive than desktop software. Let me know if clamwin will do at all what you'd like.
Clamwin is actually the one I use on my little server. I haven't figured out how to tell it that a certain file is safe, unfortunately, so I get a "virus detected" alert every time my external HDD gets scanned.