Have you checked this out?
Before you begin: If you see NavNT in the error message instead of Symantec_Client_Security, or you are using Symantec AntiVirus 8.x or earlier, read the following document instead of this one: Error: "The add-in 'C:\Program Files\NavNT\vpmsece.dll' could not be installed or loaded . . ."
This problem is fixed in Symantec AntiVirus 10.0.2 and Symantec Client Security 3.0.2. For information about how to obtain the latest build of Symantec AntiVirus or Symantec Client Security, read Obtaining an upgrade or update for Symantec AntiVirus Corporate Edition or Symantec Client Security.