I bought the license of Avast! 4 Professional Edition Upgrade for 2 years on last December but I had a problem with my computer and should format it and now I don´t know how to install the Avast again. Can you please help me with this doubt?? I should have a new Key for that? Many thanks.
Welcome to the forums, Milton Lucato 
You can send an email to … sales (at) avast (dot) com … and explain in the email in detail what happened.
Be sure to include the email address used in December if it is different than the one you use to send the email to avast.