Ubuntu Software centre not allowing install- says unauthenticated sources- GPG key problem- The action would require the installation of packages from not authenticated sources.

Go to System->Administration->UpdateManager->Settings->authentication:
remove all keys->restore default
Thats it.

Also check the solution on : http://ubuntuforums.org/showthread.php?t=1327907
It says:

johnofwax First Cup of Ubuntu Join Date: Feb 2010 Beans: 3

Re: Updates and Package Installer Problem Posting to help others: I just received the same error message. I used apt-get on the command line to install what i needed, but I didnt want to have to revert to that all the time. I like browsing the software manager for games and stuff. So after installing 2 packages on the command line, I went searching for the issue. My steps were: System>Administration>Software Sources I had "Proprietary Drivers" and "Software Restricted by copyright or legal issues" checked. I unchecked the two of these. I then went to the Authentication tab. I clicked "restore defaults", although nothing changed. I closed the window, and tried the Software manager again. Worked like a charm. Don't know how the settings were changed in the first place, but whatever. Hope it helps somebody browsing, since at this moment, this is google's #1 result for the error message.