My laptop came with Windows 8 standard preinstalled and I activated it easily. However, last week, my younger brother tried to upgrade my laptop to Win 8 Pro using a serial key he got from his friend. Now Windows is asking for activation, which keeps on failing. After coming to know from my bro that it was not a legitimate key, I decided that I should revert to the pre-installed Windows 8 standard licence and activate it. But since Windows 8 standard key is embedded in my system, I cannot find the legitimate WIn 8 standard key.

You will need to use the recovery partition to restore your computer to its original factory state:

If you would like to reinstall Windows 8, which means removing apps and personal files and settings, you can use reset and refresh. This is equivalent to a clean install.

To do it, do the following:
· Press Windows key + i on your keyboard to show the Charms bar (If you are using a touchscreen: Touch the right edge of your screen and swipe your finger to the left)
· Click Settings
· Click Change PC settings
· Click General in the left column
· Under Remove everything and reinstall Windows, click Get started
Then follow the instructions that will be provided on the screen to reinstall Windows on your PC.