I would start by reading the Help file in the Avast directory or from windows Start, Programs, Avast, Help.

The type of files to be scanned/excluded can be adjusted/amended in the on-access Standard Shield (click on the Avast icon).

You can also change the degree of scan from the Start Avast Antivirus (right cilck avast icon on system tray). Once displayed you can a icon bottom left of window and set the sensitivity of the resident scanner.

But I would start with the help file.