Any chance you have an older version of Microsoft Streets & Trips (or MS Autoroute) installed?
It contains one file in CHM format, containing over 600,000 files - so, it takes very long to process and creates an enormous amount of temporary files in between (that are not deleted if you kill the scan, instead of letting it finish).
The problem with the temporary files (and with displaying the scan progress) will be fixed in the next version of avast! (you can try the beta) - but scanning this particular file (Usaarts.its) will still take pretty long (lets say an hour, but it depends on your computer, of course) - so I suggest to put this file to the list of avast! exclusions so that it's not scanned anymore.