Error code 0xC004C020 when activating windows

When activating windows using a MAK key, if you receive the error code 0xC004C020 it means you have ran out of activations using that key.

You can login into the Microsoft licensing website to check how many activations you have remaining on your MAK keys, and also find the contact information to get additional keys if required.
https://www.microsoft.com/licensing/servicecenter/

Further information on activation error codes can be found here:

http://support.microsoft.com/kb/938450