I want to finish scanning without any annoying window
What do you mean? You mean that if a virus is found, the scanning should continue?
Well…if yes…you get some nice information about the subject in this thread:
http://forum.avast.com/index.php?topic=28402.0
And by the way…Welcome to the forum! 
These advanced automated options are only available in the Pro version.
You only have a limited option in the Home (free) version, so it requires interactive response, there has to be some difference between the home and pro versions (the programmers have to eat ;D).
In the Home version you can check the option “Don’t show this window again” when the first virus warning appears, select the “No action” button. This way, no action will be taken and you will given the results at the end of the scan (and you can perform actions from there).
However, you have to wait for the first detection, over time this will become less of a problem as the resident scanners (web and internet mail shields, etc.) will be trying to keep the infection off your HDD, so in theory there should be little to detect.
You also don’t say what settings, scan sensitivity or whether archives are scanned, that will have an impact on what is scanned ?
@ Bluesman
I believe by the comment, ‘finish scanning’ Devware is talking about an on-demand scan rather than on-access (which that link covers).
Thank you
No problem, welcome to the forums.
I assume you are using avast Home and doing an on-demand scan.
Yeah…could be so, my mistake.
No problem it also shows other options if Devware doesn’t like interruptions in general.