This sound like a Boot time scan. Which scans files piror to windows starting up. If this is to turn it on once again do the following: You need to do this everytime you want to do a boot time scan, which you do not need to do it often.
Step 1: Open Avast! Icon in the destop
Step 2: Go to Menu, which is in the top left corner.
Step 3: Select: Schedule boot-time scan
Step 3: A window should open select the things you want to scan and then click ok, it will tell you to Reboot the Comp to start the scan.
Hopes this helps
Bye