Welcome to forums !
First of all, until you provide more information no one will be able to help you. We don't know anything about your computer, type of internet connection, how your Update settings are configured in your avast!... nothing. We don't have a crystal ball, so you have to help us, and then we can help you.
It's not the same if you are connected through the dial-up or you're on broadband connection, so please provide more details and I'm sure you'll get your answers soon.
Cheers !
Edit: these are the sections where you have to look for the fix for your problems... when you tell us what kind of connection you're on, we will tell you how to set those settings...
Click on the thumbnail to see full size screenshots