Hi, welcome to the forum.
Please advise your operating system, and any other security applications active.
Did you have any other AV installed on this computer in the past?
Please check your system time and date.
The registration form for the Home version is
here, the FAQ's for registration issues are
here. Let us know if this solves anything.
You would normally fill out the form and Alwil will then email you the key, which should work, assuming it is entered correctly. (Right click the Avast icon, select "about", select "license key", enter the details, click "OK".)