Open avast's log file and let us know at what file (and the location of the file) avast seems to be hanging.
When going for a full system scan, I suggest you run
CCleaner first to remove a lot of culprit.
There is no need to scan temporary files and such.
Lesser file to scan = shorter scan time.
Run a boottime scan, that way avast will run with a minimum of other things running (in the background) and can make better use of your system.
That also shortens scan time.