Please refer to the following Symantec Knowledge Base article:
<http://service1.symantec.com/SUPPORT/nav.nsf/3353489cd5cd08c388256c0e005a96e4/46df6663b5250b3c88256c52006caeab?OpenDocument>

This article clearly explains the cause of this issue and provides a detailed resolution. For your convenience I have listed the steps below:

1. Uninstall NAV by using the Rnav2003.exe removal utility.

Removal Tool Download Link:
ftp://ftp.symantec.com/misc/consumer/Rnav2003.exe

2. Delete the NAV and virus definitions folders and the virus definitions registry entries.
3. Reinstall NAV.

Related Knowlege Base article(s)
===============
Title: Error Message: The System Administrator Has Set Policies to Prevent This Installation
<http://support.microsoft.com/default.aspx?scid=KB;EN-US;322963>