I am not sure if this has been posted, but I am having trouble finding the scheduled scan function. All the past AVs I have used have these… does Avast not?
Chris
Hello and Welcome 
Scheduling scans are available only in the Pro version of avast!.
If you are already using the Pro version, just switch to the Enchanted User Interface and there you’ll find the scheduling option 
EDIT: if you have the Home version , you can see HOWTO: Scheduled Automatic Scans w/ Free Version!
If you have the Home version, I’m not surprised you couldn’t find anything it isn’t their. You can use the workaround XMAS posted, but with the various resident providers Web Shield, Internet Mail, P2P Shield, Instant Messaging, etc. all working to detect incoming viruses and Standard Shield as a fall back line of defence, the need for on-demand scans is lessened.
I do a weekly on-demand scan of Local Drives, Standard sensitivity and without archives, this doesn’t take long so personally I don’t see the need to schedule scans in this way.
Welcome to the forums.
Thanks all… yeah I just have the basic trial version, but I like tis program. It caught my first virus hassle free the other day, without any fault. How much does it cost to upgrade?
Follow the links here.
http://avast.com/eng/how-to-buy-avast-professional-edition.html