I also put forth my vote for AVG. Though my own workstations are Linux based for all my customers running Windows it's AVG all the way. I have installed it up to a network of 100 workstations and it can scale further if needed. Of course the 20 workstation scenario is covered quite well by it. Like it has been mentioned it doesn't require a server to be installed on. Also please note that they also have linux clients available as well. Oh and versions exist for File Servers as well as some mail servers.
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I also put forth my vote for AVG. Though my own workstations are Linux based for all my customers running Windows it's AVG all the way. I have installed it up to a network of 100 workstations and it can scale further if needed. Of course the 20 workstation scenario is covered quite well by it. Like it has been mentioned it doesn't require a server to be installed on. Also please note that they also have linux clients available as well. Oh and versions exist for File Servers as well as some mail servers.