Welcome ena,

First to answer your questions:

1)When I have uninstall the program, must I use the original setup or dld I a new one?

You can use the original setup if you want, also make sure you reboot the pc as you uninstall avast, then reinstall.

2)What do I whit the license, can I use the same as present?

Yes you can.

But before you reinstall avast, have you tried a repair of avast yet?, it is much faster and more convenient.
You can get to it by going to Start>control Panel>Avast>Change>Repair.

–lee