There is a few different ways that you can activate Windows 8.
The first way is to do a search for Slui.exe 0×3. Run the Slui.exe 0×3 command in search and click the Slui.exe 0×3 icon.
Then type your product key in the Activation window then click on the Activate button.
The second way is to run a command from the command prompt.
Make sure you are logged in as a Administrator then do the following.
In the search screen type CMD, which is short for command.
Press enter or click on the command prompt.
Then type the following command.
Cscript.exe %windir%\system32\slmgr.vbs /ipk “Your product key”
This will use the serial key you enter and activate your copy of Windows 8 with the key you supply.