I installed Avast free, regestered and was e-mailed the following:
Your avast! Free Antivirus registration was successful. Your license key must now be inserted into the program.
Your license key is:
---------- Cut here ----------
W03118172H1200A0617-6D79J8FT
----------Cut here ----------
I followed the instructions but an error box popped us stating that the “license key is not valid”.
How do I get a valid key?
Thanks 
How to register https://www.avast.com/en-eu/faq.php?article=AVKB9#articleContent
Avast FAQ https://www.avast.com/en-eu/faq.php
Avast FAQ videos https://www.avast.com/en-eu/faq.php?q=Video
Thank you for your quick responds.
I have already followed ALL your links and an e-mail giving the license key. Avast says the key not valid. What do I do now?
Your link does not help. I have already been thru ALL the steps. Key still does not work.
What do I do now?
You should not need a key when register avast free
Avast support https://support.avast.com/
There is no such thing as a license key.
There is a activation key and a license file.
In an indirect way there is, if you use the off-line registration process (not through the avastUI), you then receive an email with your registration key.
That said:
@ crgibson
If you copy and paste that key, ensure that you don’t copy any trailing spaces.
You can register directly from the avastUI > Settings > Registration - if you have register this way - that should be it done and dusted. The Insert activation code or Insert license file options are if you wish to upgrade the free version to one of the paid versions.
I did register thru online. I received the following e-mail:
Dear avast! user,
Your avast! Free Antivirus registration was successful. Your license key must now be inserted into the program.
Your license key is:
---------- Cut here ----------
W03118172H1200A0617-6D79J8FT
----------Cut here ----------
To insert your license key, please follow the instructions below:
If you experience any trouble while inserting the license key, please click here for more help: https://www.avast.com/en-us/FAQ/AVKB9
** Important notice **
Please note that, under its license conditions, avast! Free Antivirus is for home, personal, and non-commercial use only. Commercial versions of avast! can be found at http://www.avast.com
Thank you for choosing avast! antivirus.
The AVAST Software team
http://support.avast.com
http://forum.avast.com
I followed the instructions as listed (I have tried this 4 times) and still get the license key is not valid error.
Should I uninstall and register under a different user name & e-mal and password and try again?
If you experience any trouble while inserting the license key, please click here for more help: https://www.avast.com/en-us/FAQ/AVKB9
Click the link https://www.avast.com/en-us/FAQ/AVKB9
Scroll down to the blue button Detailed steps click the button and you find instructions for online no key register
I went thru all that BEFORE I first posted. (twice).
Next
Can you take a screenshot of the avastUI > Settings > Registration, like the one I posted and attach it.
This will show if you are already registered, because if you aren’t there should be a different screen to the one I posted to allow for the manual input of the free version registration.
Having posted your key twice, it can be harvested and that could well end up being used by many others. This in itself could result in it being blocked by avast leaving you blocked also.
Also did you try what I said, ensure there are no trailing spaces when trying to enter the registration key ?
registration key has no spaces in front of, or at end.
Pondus -
OK. I used your link and was sent a new license key. I followed directions and got the error “License Key is not valid”. (again).
I followed the link given in the e-mail for more help: hhtps://www.avast.com/FAQ/AVKB9, which took me to the web page you listed in your reply #9 about the “blue button”.
I followed instructions for online no key register. I got the error 0x80046400 box.
I’am back to square one. What next?
There is nowhere for you to insert it - as I mentioned in my last post (Reply #7):
The Insert activation code or Insert license file options are if you wish to upgrade the free version to one of the paid versions.
Entering the free registration key in either of those options will fail.
If you tried the Offline Registration field ‘Insert License Key’ and are still getting the invalid key error. You could try to get another as Eddy suggests, but personally I would try something else as that would result in another email with a registration key and you haven’t had any luck in that.
What happens when you use the big Register now button ?
That should open a different input screen and the registration happens within the program, no email to wait for and no manual input.
I think that I don’t have a problem after all.
I just went to Avast user interface/Account. It shows Subscription Status: Valid. (see image #1)
However the Avast tray icon has a red X on it’s face. The white flag icon (Action Center) also has a red flag on it. Opening Action Center (see image #2) says to "Turn on Avast Antivirus. Nothing happens when I click on box “turn on”.
What OS/SP ?
What exact verion of avast is installed now ?
Do you have any other security software installed ? (or did you)
To fix the notice that Avast needs to be turned on I suggest you REBOOT your computer. 
Well you clearly do have a problem as the avasu installation doesn’t show you are registered when my.avast.com does. Since you are using the UI it is important to resolve the problem and get avast running also.
Try a repair of avast:
XP - Add Remove programs, select ‘avast! Anti-Virus,’ click the Change/Remove button and select Repair, click next and follow.
OR
Vista, win7 or later - Control Panel, Programs & Features, uninstall a program, select ‘avast! Anti-Virus,’ click the Uninstall/Change and select Repair, click next and follow.
You should reboot after the repair.
This has in the past resolved this out of sync issue on data displayed (or not displayed) and the data behind the scenes.