The default is ‘disabled’… but probably by accident you have checked the option ‘Show detailed info on performed action’ in the Standard Shield settings (tab Advanced).
But if you want an advanced way of getting transparency of this popup window, see here at section [AAVM]:
TagOpacity=70
Semi-transparent feature of avast! On-access scanner (if Internet Mail Provider is configured to “Show detailed info on performed action”. The number represents a percentage: 100 is fully visible and 0 is fully transparent. A reboot may be required for the changes to take effect. 