For Windows server, these days there are number of good alternatives available. Some of these are :

1. Kaspersky Anti-Virus for Windows Server
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2. avast! Server Edition - antivirus software for Windows Servers
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3. Panda Antivirus for Windows Server 2003

4. AVG
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As i haven't used any of this on a server, i cant tell you which is best. Just give a try any of them u like.