Licence Key Invalid

I have just downloaded avast! antivirus onto my computer. I recieved an email with my licence key so i copied and paisted it for registration and a pop up comes up and says invalid licence key. I copied and paisted a few more time just in case i made a mistake but the same thing just keeps popping up. So i registered again and got a new licence key, and when i copied that one into registration it again pops up as invalid. I dont know what to do, but i am in great need for a working antivirus as i know there is something wrong with my computer and want to fix the problem asap… any help would be muchly appreciated. Thanks!!!

Did you use the trial version and now you’re using the Home? I mean, did you downgrade from Professional version to Home?
Are you copying the last blank space at the end of the key? If you write one by one the key characters, will it work?