1)When I have uninstall the program, must I use the original setup or dld I a new one?
No, you can uninstall through Control Panel.
To reinstall you can use the old setup and then update or, better, download a new setup.exe file.
2)What do I whit the license, can I use the same as present?
License is valid for updating for 14 months.
You can use your old license or, if you want to start 'new' 14 months, you can register avast again

About the repair failure, were you connected when you've tried to repair?