my windows 8 pro it was active first time wen I install it and then I change the hard drive and install it.

Now it won't active can you help me please. 

Have you tried to reactivate by phone?  Not sure if this is available yet or not.  You could also try to use the slui.exe 3 option in the run dialogue box.

From an admin command prompt type:

slmgr /ipk YOUR-ACTIVATION-CODE

Thanks Systems_UK! My Windows 8 was not activating in the proper method (with the GUI). Your command worked first try, thanks!

Dear

when I ran this command it showed an error

"Error:0xC004F050 The Software Licensing Service reported that the product key is invalid"

let me update you that I was using Window 8 Preview that I got activated successfully. Then I installed Win 8 Pro on the same partition after formatting.



In SEARCH enter sliu.exe 0x3 = opens an activation window that allows you to enter a full activation key