Once you select the avast! antivirus as yoyr screen saver click the Settings button as that many options.
Areas: default - all HDDs, you can be more selective.
Types: default - Scan default extensions (same as the Standard Shield extensions) and you can add others.
So these two basically say it isn't the same as the ashQuick.exe nor the Thorough scan (scans all files) of the Simple User Interface (on-demand) scan.
There are many other Options you can configure.
So this scanning option is much more configurable and flexible than either of the scans you mention.
Check the Virus option on the avast! antivirus settings as that you can configure to deal with virus detections. Deletion should be in my opinion never used as an option (you have none left), I would only use the Move to Chest.
However, I don't use this feature I prefer to do a manual scan (Standard sensitivity, without Archives) with my regular weekly maintenance tasks (this doesn't take long, but my system isn't chock-a-block with data).