One or more options/settings in an ever-growing number of third-party
applications may be disallowing the change(s) from "sticking". These include
but are not limited to Ad-aware's Ad-Watch, Spybot Tea Timer,
SpywareBlaster, SpySweeper, Spyware Doctor, Norton AntiVirus, McAfee
VirusScan and/or Antispyware, and Zone Alarm (Free and Pro).
Here's something else about NAV that you may not be aware of:
http://bertk.mvps.org/html/symantecdoc1.html