Windows 2003 Cal for an Antivirus Server

Hello,

I just got on board a company that uses a Windows 2003 Server for its Sophos Anti-virus Software.

I have a Linux based PDC server so I dont need active directory.

The current Windows 2003 Server has 5 CAL’s included but there are 100 Windows XP Pro with Sophos anti virus client accessing this server.

Question: Do I need an additinal 95 cals for all of the windows xp machines?

Sorry for the newbie question. Im a Linux Administrator so Im not too sure about Windows CAL’s