On this screen you can login to an existing Avast Account, or create one by clicking Create an account now in the bottom right corner.

The Avast Account is a self-service management tool. Once logged in, the Account section provides access to the web portal where you can:

If you want to connect your computer to a different Avast Account, simply log out and you can then log in to another account.

Your Avast Account is automatically created when you register your Avast Free Antivirus. Account registration is optional for users of paid editions.