This is a method employed by Microsoft to verify that the copy of Microsoft Windows installed on a computer is a legitimate copy. Sometimes Microsoft performs this verification before allowing that computer to download any updates or additional software from Microsoft.

Older versions of Microsoft Windows, e.g. Windows 95, 98, and NT 4.0 do not require any type of WGA for downloading and installing Windows updates, however Windows Genuine Advantage is built into Windows XP and Windows Vista. 

Some false results that report legitimate copies of Windows as pirated copies have made this a very unpopular system for the average computer user.