Deactivate windows 8 and 8.1 keys for reuse

Press Windows key + X

Click Comman Prompt (Admin)
At the command prompt, type: slmgr.vbs -upk

Hit Enter, this will uninstall the product key from the computer and set it back to trial mode, you are now free to use it on another computer. If you encounter any problems, try activating by telephone:

1.       Press Windows key + X then click Run, then type:   slui.exe  4

2.       Next press the 'ENTER' key

3.       Select your 'Country' from the list.

4.       Choose the 'Phone Activation' option.

5.       Stay on the phone (do not select/press any options) and wait for a person to help you with activation.

6.       Explain your problem clearly to the support person.

The installation currently will go into trial mode, will need to be activated or it will go into reduced functional mode. You have the option of installing another version or edition of Windows or obtain a product key for it. You can also format the system.