Actually, although this does seem to work for an automatic scan it takes forever. You would almost have to set it to run overnight.

I tried it a couple of days ago and it seems to scan every single file on the drive. I stopped it somewhere in the middle of my mp3 files in my music collection - after about 50 minutes it had scanned just over 6000 (yes, six thousand) files in that time.

The complete manual scan I had done just after installing Avast looked at something like 57000 files in about 20 minutes - much more like it!