Several websites are publicising another way of bypassing Windows' new anti-piracy system, Windows Genuine Advantage (WGA).
The technique dates back to a posting on the insecure.org website in May, and involves changing a setting in a Microsoft validation tool, GenuineCheck.exe.
This generates a code that can be used to validate pirated Windows software.
A Microsoft spokesman said, "It is not a surprise for us that those who never intended to pay for software would try to find some way to circumvent Windows Genuine Advantage."
WGA requires users to confirm the serial number of their Windows package before updating it online. But hackers claim it took less than 24 hours to break through the security measure aimed at excluding users of counterfeit software.
Two weeks ago, Microsoft made the Windows piracy check mandatory for all customers who want to download add-ons for Windows XP and 2000.