If there is any NIS symbol in the system tray, rightclick on it and exit the program(s).
Use Explorer to do a file/directory search, and delete any and all directories with names like NAV, NIS, Symantec and Norton.
Go into Regedit and make a full backup.
Start a Find Norton and after that a Find Symantec.
Delete any keys you find. Press F3 for the next find. It's a tedious job (especially the CLSIDs), which will take you the better part of an hour.