I had the same problem a few weeks ago at work.

I had to contact symantec who gave me a link to there FTP iste to download SAV cleansweep, it is a utility that completely removes the total existence of NAV from the system.

They wre very helpful and gave me access to it quickly, however we do pay for support from them so not sure if you can get it without support, ontact them to see ?

Failing that there are tons of registry cleaning tools out there that will do the job i am sure