Windows Genuine Advantage (WGA) is a feature of Microsoft Windows Update that will check to see if your copy of Windows is valid.
If it detects a invalid or pirated copy, it will not let you download any updates except for the critical ones. That means no software updates, no hardware updates, nothing.
Something you may not know (cause Microsoft neglects to tell you) is that WGA sends information about your computer, what software you have installed, your IP address, etc. without your knowledge on a regular basis. So when did Windows become Spyware?
This is what Microsoft had to say about it:
"First of all this is a pilot - customers have the choice to subscribe or not. WGA is very careful about which license keys are checked - some numbers have been leaked and therefore have been culled by Microsoft. If customers bought a genuine copy of Windows but as a result of a poor installation or a repair a different license key was used then WGA would flag it as not genuine."
If your copy of Windows is deemed invalid or pirated, WGA will quickly shut down. Yet, does that stop Windows from popping up that annoying little reminder balloon by your clock tell you to update? They need to have a option that you can select that says "Yes, my copy of Windows is invalid/pirated, please leave me alone about it!". More...