Use BleachBit + Windows Disk Cleanup for 90% of the benefit of Avast Premium without paying a cent or risking a license file.
Note: If you have an activation code (a string of letters and numbers) instead of a file, you can paste that into the same window, and Avast will generate the license file for you automatically.
Avast will instantly verify the file. Once confirmed, the interface will change from "Free" to "Premium," and all locked features (like scheduled cleaning and browser cleaner) will become available.