A great many Legacy users with the Avast anti-virus program cannot start Legacy after updating. The solution is to re-install Legacy. You can download the most recent version of your Legacy Family Tree program, free from the following link:
Previous Versions of Legacy: Click Here and download the final release (build) of your version.
One Legacy user explained "Issue resolved. My anti-virus program simply did not like the Legacy.exe file in the download and quarantined it." She added Legacy.exe to the allowed or safe list and Legacy worked. Please consult your anti-virus program's help file for the steps.