ClamWin Anti Virus
All the ones mentioned above have free versions with free updates. I am currently using AVG AntiVirus and find it to be a great anti virus program.
I use ClamWin on the older computers that I have. Be warned that it does not have an on-access scanner. So if your organization is constantly bringing in external files, then it wouldn't be the best for you to use.
What is everyone's suggestions for a free antivirus for a small church. It's a stretch for us to afford entry level computers. So anything free is much appreciated.