Download Windows 8.1 Activetor

Advantages of activating Windows:
-Full use of your PC
-Ability to not worry about it.
If you don't activate it, the end result is somewhat unpredictable and seems to depend on the product channel(retail, OEM, volume). Either it will go into 'extended evaluation' mode, where you'll get the black wallpaper, the "this product is not genuine" watermark in the corner, and reboots every 30 or so minutes. Or if you're unlucky it will completely deny access to your computer.

If you have the product key to do so, activate. It's not worth the hassle. If you don't have one,then download the software and active your windows 8.1

watch this video instruction for active windows 8.1..