The nac.dll file is part of an Internet Audio Network Access Control (NAC) module for Windows NetMeeting from the Microsoft Corporation. Windows NetMeeting was a peer-to-peer messaging client included in many versions of Windows (up to XP). It allowed for videoconferencing and VoIP communications.
Network Access Control is a security measure that protects your system from unwanted intrusions by using authentication at the system level. A problem with this file may disable your ability to use Windows NetMeeting. You should try cleaning your registry to fix this issue.
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