If you have installed windows 8 enterprise RTM, despite the fact you used a valid serial number you might not be able to connect to the Microsoft activation services. This will cause your device to display in the bottom right corner the message “activate your windows 8 copy”. Here’s another way to activate your Win8.
Call the command prompt and run as “administrator”
in the command prompt call the SLMGR utility with your serial key
The operation might take a few seconds and when is complete you will see the message below and your windows 8 enterprise copy will be activated.
Update: Yet Another Method.(tip from Glen Maroney)
Another way to do this is also calling SLUI 3 from the command prompt.
And from there paste your serial key and click activate.