WGA stands for windows genuine advantage. It is created by Microsoft to stop piracy. When the message pop-ups in your desktop then it means that your license key is invalid. It will provide you a grace period. After that your system will not work. But if you had an original windows version and then also you getting trouble the you can manually stop it. To disable it go to Start Menu > Control Panel > Add remove program. Look of Windows Genuine Validation and click on " Click here for support info " tab. In the box verify the version number and select close. After that go to Start Menu > Run > type cmd and hit enter. Give the following commands :
Restart your system.