The best, free anti virus out there right now is Panda. This thing is kicking the butt of most paid apps. Why anyone would pay for anti virus is beyond me.
With that being said, I realize this is a business setting. Looks like Panda can be used in a non profit organization.
Found this:
http://www.geekout.biz/2013/06/17/fr...nd-nonprofits/