Use an alternative administrative account
You may be able to authorize installation of Avast software by entering credentials for an alternative user account with administrative privileges. When the User Account Control dialog appears, click More choices to see a list of administrative accounts on your PC, then select an alternative account. If you only have one account with administrative privileges, the More choices option does not appear.
Reset your administrative account password
In some cases, you may be able to reset the password for your administrative account. To learn more, refer to the relevant Microsoft support article below:
If neither of the solutions above resolve your issue, contact your device manufacturer.
Source : Official Avast Brand
Editor by : BEST Antivirus KBS Team