1. How to fix Avast Antivirus Error 42139

2.  First of all, you need to click the “Start” button.  In the search box, type "command."  Hold down the CTRL-Shift on your keyboard, and then hit “ENTER.” Repair Registry Entries Associated with Error 42139:

3.  Here, you will be prompted with a permission dialog box, click “Yes” for further preference.  Now, you will see a black box with a blinking cursor.  Next, type "regedit" and hit “ENTER.”

4.  Select “Export” from the File menu  Select the folder where you want to save the Avast backup key in the save list.  Type a name for your backup file, in the File Name box.

5.  Now, make sure that the “Selected branch” is selected and then click “Save.”  The file is then saved with a.reg file extension.  Finally, you will have a backup of your Avast! Antivirus-related registry entry.