I don't know about NAV or the 2008's any more but in NIS 2009 it is under Computer / Settings and at the top of the listing.
Note that for 2009 anyway you may run into long delays in scanning if you have scan compressed files as the default. This is because it can take it a long long time if the archive has many layers of compression -- files within files within files.
Symantec have explained that you are still protected with it set not to scan inside compressed files because it will scan the moment you go in to extract one.
FWIW bearing in mind that I don't know about NAV 2008 specifically.
Norton AntiVirus scanner can scan upto 10 levels of compressions. Also, it can scan some archive file types such as .cab, .arj etc but cannot modify them i.e. it will detect the virus infections in those archive files but cannot repair or quarantine them as these types of files does not allow the modifications. This won't happen when you download the compressed file from the Intenet. When you extract the files from the compressed format after downloading it, Norton program will scan it and report if it finds any threat. If you are not extracting it that time, then Norton program decompress and scan that file when you run a full system scan or when the idle time scan runs.
You cannot free upgrade from NAV 2008 to NIS 2009, you can only get free upgrade to NAV 2009.