This is from my NAV 2011 help file:
By default, the Intelligent Skip Scanning option is turned on.
When Intelligent Skip Scanning is on, Norton AntiVirus does not scan some of the common non-container files types that are available on your disk. The commonly available non-container file that Norton AntiVirus skips are .dll, .gif, .png, .jpg, .asf, .avi, .bmp, .htm, .ini, .css, .cur, .fon, .ico, .inf, .sys, .ttf, and .wav.
If a container file appears in a non-container file format Norton AntiVirus considers it as a non-container file and skips the file from scanning. However, Norton AntiVirus scans all the files that you receive through email even if the Intelligent Skip Scanning option is turned on.
When you turn off the Intelligent Skip Scanning option, Norton AntiVirus scans files of all types, regardless of their extension.
Dave