Well I guess your weekly back-up probably also changes the file properties, like the last accessed date and or some other indication for your back-up program. So it may be that it opens the file for writing which may be causing avast to scan the file first.
You could pause the standard shield before the back-up so there is no interaction, rather than completely de-active avast. I usually run a scan before I do my weekly Drive Image back-up so if you did a similar thing then pausing the standard shield wouldn't be so bad. Though I have seen no change in my weekly drive image back-up as it obviously operates at a low level, it takes minutes rather than hours, but my data is much less than 38GB.
I back-up all volatile date files, the things that you don't want to lose, data files, like documents, spreadsheets, emails, email account details, registration keys, address book, favourites/bookmarks, downloaded files/programs, etc. the list goes on and on but if you don't want to lose it back it up. There are many back-up programs that can simplify this task and run it every day sometimes more frequently. avast doesn't seem to get in the way of that.
So if I do experience a serious crash at worst I lose 7 days worth of program installed (but should be able to re-install them from daily backups) and 1 days worth of volatile data.
I use a little program called mirror and I create a batch file to back-up those files I mentioned, it copies them to my second HDD and keeps the two locations synchronised so if I delete something from the source location it also deletes it from the destination.
