I have the same problem as him (but I really don’t care about the high CPU usage).
Since Steam download games in many small parts, I can understand that Avast! is scanning all of them (just take a look at the “Statistics”).
I know it’s not the best solution because of the less security. But still, I don’t know anything else that will work.
Maybe repair or a clean installation will work.
EDIT:Oh, please note that I didn’t made that suggestion because I wanted to be mean. Just wanted to help.