Thanks Tech and DavidR,
I think I worked out the reason it was happening. Since it was a new installation of avast, all of the files on my computer were considered new, therefore it attempted to scan them all. Unchecking open/created/modified files did stop the background scans, but I decided not to uncheck these as I agree this is a useful function.
I did a full system scan yesterday (my first) and when I booted up this morning, avast is no longer attempting to scan in the background, since it is now aware of the files that are present. I have left the Standard Shield on normal, and it is now working exactly as I want it to.
Thanks,
David