If your copy of Windows is not genuine, you will get an error message and Windows will no longer work. There is an easy fix you can apply in minutes that can get rid of this error message until you purchase a legit copy or solve your product key issue in Windows.

Click on Start

Type cmd in the search box. (Optionally go to "Start, All Programs, Accessories, Command Prompt" in Windows 7, others will be similar)

Right-click on cmd and choose “Run as administrator”

Type in SLMGR –REARM and hit enter. If it fails, try SLMGR /REARM (Only difference is / instead of -)

If it worked you will be told: “Command completed successfully.”

Reboot

Now you will not see the message again for two months.

If you do this too many times, you will get an error 0xc004d307 "The maximum allowed number of rearms has been exceeded.” You will need to edit the registry to get around this.

Go to HKEY_LOCAL_MACHINE - SOFTWARE - Microsoft - Windows NT - CurrentVersion - SoftwareProtectionPlatform
Right-click on SkipRearm. Click Modify. Change “Value Data” to “1”. Restart your computer.