Sorry this could not happen :cry:
The application must keep active in the background (with the blue ball at system tray).

Did you have, at least, a blue ‘crossed’ ball at the system tray?

No, there isn’t an standalone on-access scanner to be installed…
Did you try the repair option on the Control Panel > Add/Remove programs > avast antivirus > Change/Modify and then choosing ‘Repair’ at the pop-up window?

Which is your OS?