Customers who find themselves reinstalling Windows XP should be ready for a headache: Microsoft will no longer support activating the product over the Internet for PCs which have Windows pre-installed.

Intended to curtail the stealing and selling of certificates of authenticity, the new security measure will start at the end of this month. At first, it will be limited to the Windows XP software preinstalled on systems shipped by the top 20 PC sellers. "The main reason (for the change) is to address piracy in this area," a Microsoft representative said on Thursday. "Microsoft has found various people selling the labels of authenticity that they have copied or have pulled off other PCs." The change is the latest attempt by Microsoft to target software pirates who try to sell stolen copies of Windows XP or the certificates of authenticity that mark the software as legitimate. The company has a plan to check that people's operating systems are properly licensed before allowing them to download certain updates. The plan, known as the Windows Genuine Advantage initiative, was introduced in January.