Hello all I have been using this product for years without a hitch…once putting in the license key no problems…so now I have a issue every time i reboot it come up with a red flag for me…software is about to expire so I put the license key in and it says successfully installed…Does anyone have a clue? 
How many administrator accounts do you have in this computer?
Is the system time and date correct?
One administrator, todays date and good time
Which program are you registering? Home or Pro version?
If you’re using the Home, can you register again see the tutorial: http://www.avast.com/files/tutorials/insert_key.htm
Please take a look here: http://www.avast.com/eng/faq-invalid-license-number.html
How long did you use the trial version?
How many times did you install/uninstall the trial version?