It would help to know which OS you are using and if you have managed to get rid of all traces of NAV.
1. clear your temporary internet files, IE properties, Temporary Internet Files - Delete Files.
2. run another scan with avast and if given the option either delete or move file to chest.
3. If you have WinME or XP, disable System Restore and reboot.
4. scan again with avast, if clear enable system restore.
5. if not clear, open the avast scanner and from the menu, select, 'Schedule boot time scan,' exit the program and reboot. This will repeat the same scan that happened when you first installed avast, since it is run before windows is activated it should get rid of most problems.
6. when you get in confirm you are clean.
That should keep you busy for a while, you might also want to read the FAQ thread in the Off Topic Forum.