No, you cannot use more than one antivirus (generally speaking) because they will conflict each other. avast is not the only one which conflicts with NAV.

Click on High sensibility of scanning or, better, the Custom level and choose scanning ‘all files’ including archives.

Which ones? Can you post their names?
Which files (and path) were infected?
Can you send them for analysis to virus@asw.cz ?

See more about Norton here: http://forum.avast.com/index.php?board=2;action=display;threadid=4410;start=0#lastPost

or search the board. Wellcome :wink: