Hello, Marius. No, OneCare for Server will not run on Server 2003 R2. It is designed to Server 2008 64 bit edition and is meant to be used with SBS 2008.
There are many other security products that will run on Server 2003. OneCare can be installed to the client PCs, but 9 to 12 computers will require 3 to 4 subscriptions - each set of 3 PCs using the same Subscription LiveID. The clients will not interact with the server install of other security software.
Microsoft offers Forefront Security, but I suspect that will be overkill for your environment.
-steve