Windows 8.1 is the Latest Version of Windows. If you have purchased the Licensed version of Windows than you don't need to use Activator . You can use your own registered key to Activate windows or You can Activate windows Online.
If you have the pirated version of windows then Activator is the best choice to Activate the Windows



KMS Activator for Windows and Office

Capable of Activating:
    Windows 8
   Windows 8.1
   Office 2010
   Office 2013

How To Use-

Double click on KMS 8.1.exe to run the activator
Choose your Product: Windows, Office or Windows and Office
 Press either 1, 2 or 3 as your requirement.
Wait for the operation to be finished.
 After successfully installing the KMS, you will be confirmed.
 Now close the window and check your activation status from System Properties. (Hit Start + Pause)


Note: If you're trying to install this KMS on your PC for the first time,
you maybe required to allow any program like adapter or something like that.
Don't worry. Just allow it.