I also saw a window with the message below:
When I clicked on the "Go online and resolve now" link, I was taken to a Microsoft website which displayed the message below:
Enter a genuine Windows product key.
The product key used to activate Windows on your PC is already in use on another PC or has been blocked by Microsoft. Not to worry, we can help you with that.
After the purchase of a new product key, I clicked on the Start button, then right-clicked on Computer and selected Properties . At the bottom right-hand corner of the window next to the existing product key was a link to "Change product key". I put in the new product key, but during the verification process still received a message that the copy of Windows might not be valid.
Firefox was the default browser and was apparently being used for the verification process. I noticed that the "Windows Activation Technologies Plugin for Mozilla" was listed under "Unknown Plugins". I made Internet Explorer the default browser, so it would be used instead of Firefox for the validation process. I then entered the product key again and this time I saw an "Activation was successful" message.
I then tried installing all available updates. Again, I saw the error message "Code 643 Windows Update encountered an unknown error." But when I rebooted and ran Windows Update from within Internet Explorer again, this time I was able to successfully install all available updates.
Created: Saturday October 12, 2013