How to activate windows after the activation period expires

Step 1: Open regedit in administrator mode
In the start menu, go to accessories, and then right click on the command prompt. select "Run as Administrator" accept the user access control warning and then type regedit into the command line and press the enter key.

Step 2: Reset the mediabootinstall key
Navigate to HKLM/Software/Microsoft/Windows/CurrentVersion/setup/OOBE/mediabootinstall and Change it's value to 0 (zero)

Step 3: Reset the activation grace period
In the command prompt that you still have open,
type "slmgr -rearm" and press the enter key. (you can reset the activation period up to 4 times.)

After slmgr has shown you the dialog stating that the rearm was successful, reboot the computer.

Step 4: Activate windows
Right click on computer and select properties, In the computer properties scroll to the bottom and click on "Change product key". Enter the activation key that is on your certificate of authenticity and click next. If activation accepts the key, you are done.

Step 5: If activation was not successful,
Continue on with the activation dialog and complete the activation over the phone. You can also double check your activation key as many of the digits are difficult to tell apart. I take a picture of the key with my phone and zoom it in to tell a B from an 8 and a G from a 6.