Never disable your anti virus!
It is never wise to disable your anti virus program. If you do this, then make sure you enable it again as soon as possible. A bettter way is to use one of the number of alternative free anti virus programs that are listed here:
If you need to disable Avast temporarily then you can do by hovering over the Avast icon in the system tray area of your taskbar:
Then right-click and select Disable for 10 minutes from the shields control menu item: