Each of the last two Avast updates disabled the Windows 7 screensaver. I wasn’t sure that Avast was to blame the first time, but suspected it because it was the only change to my system before it happened. After updating to the current version, I checked immediately after rebooting and found the saver agin disabled, suggesting that Avast was almost certainly responsible. It’s a minor nuisance to have to re-enable it, but it really shouldn’t be necessary.
In case it’s significant, I rub Win7 Pro x64 and update Avast using the offline installer.