You need to use the Exclusion lists:
Standard Shield provider (on-access scanning):
Left-click on the avast-ball in the tray area, click on the provider icon at left and then click on "Customize".
Then go to the "Advanced" tab and click on the "Add" button...
For the other providers (on-demand scanning (such as the screensaver) or the Simple user interface):
Right-click on the the avast-ball, then click on "Program Settings"
Go then to the "Exclusions" tab and click on the "Add" button...
If you want it, you can use wildcards like ? and *