Please where can I find and download “aswCmnOS.dll” ?
Sincerely,
Syrose
Isn’t that file included in avast! installation?
If you had deleted this file, you can try to repair avast! ( Add/Remove programs > avast! antivirus > Repair ) 
Aren’t you trying to reinstall avast!?
Please delete its folder
C:\Program Files\Alwil Software\avast4\
first or use avast! uninstall utility.