Microsoft: Legit Windows or no updates! -ZdNet:
Aiming to crack down on counterfeit software, Microsoft plans later this year to require customers to verify that their copy of Windows is genuine before downloading security patches and other add-ons to the operating system. People whose copies are found not to be legitimate can get a discount on a genuine copy of Windows.

EDIT:
Historically, Microsoft has trod carefully when it comes to crackdowns, particularly in emerging markets. Though clearly eyeing growth, the company has not wanted to push too hard in countries where piracy is rampant, and thereby force customers toward Linux. Also, some say that by threatening to withhold security updates, Microsoft is making the entire Internet less secure, harming legitimate customers as well.

In an effort to placate that concern, Microsoft will allow those with unlicensed copies of Windows to continue getting security patches by turning on Windows' Automatic Update feature.