You really are asking the wrong people, those on the avast support forums are going to be using avast, unless they are prepared to uninstall their AV to install NAV 2009 and I don't think you are going to find many takers in that regard.
As a similar question on the NAV forums, is avast better than NAV 2009 and you are not only likely to get the post detected as totally unrelated to NAV their response is as likely to be as biased as it is likely to be here.
I certainly won't be installing any NAV on my system regardless of all the marketing hype. I'm of the opinion of if it isn't broke don't fix it school of thinking and avast certainly isn't broken so I'm not looking for a replacement.