I have been using the Free Version of Avast for a long time and have never encountered any problems. The last few Days however, the Scheduled Full Scan has been Stopping. I have it set up to Run at 2:00 A.M. every Morning. I wake Up to the Avast Dialog Box Saying that the Scan has “Stopped.” It never stops in the same place. It may be at 24%, 56%, or 97% for example. I can’t figure out what is causing this to happen. I Run a Manual Full Scan and it Completes without a Problem. Any ideas?
Hi Michael and welcome to the forum. 
- What version of Avast Free are you using? Are you using the latest version?
- What other security software do you have on the machine or had on it before?
- Why do you run a Full Scan every day? There is no need to because Avast scans (resident scanning) all the time. Most people run a Full scan weekly, if at that. Then if there is a problem, run a boot scan. Otherwise, they run a Quick scan. I would recommend instead of running a Full scan everyday that you get MBAM and have this in addition to Avast for more comprehensive and layered protection; they work differently yet are compatible with each other.
To fix your problem, try running an Avast Repair, then reboot. Otherwise, do an uninstall/reinstall using the Avast Uninstaller tool for a fresh copy of Avast. But try the above first. Let us know how it goes.
In addition to what safesurf said, check the log files to see where avast stops.
Is it perhaps always at the same file or with the same error?