Microsoft has included software in its latest batch of updates for Windows that checks whether copies of the operating system are legitimate.
Users who choose the Express Install option for Microsoft updates will automatically install the new program without seeing it listed.
However, if they are running a copy or other illegal version of Windows they will soon know about it.
"For customers who are running a non-genuine copy of Windows, the new feature will enable notifications directly to their desktop that the copy of Windows they are running is not genuine," said Microsoft in a statement.
The move is part of Microsoft's Windows Genuine Advantage initiative.
Microsoft warned that it is difficult for those outside the industry to appreciate the impact of the problem.
"The Business Software Alliance has estimated that 35 per cent of all PC software used worldwide is counterfeit or otherwise illegal," said Microsoft.
"The recent IDC Economic Impact Study asserted that if the piracy rate was lowered by 10 percentage points over the next four years, this would contribute 2.4 million new jobs and $400bn in economic growth to the global economy."
The notifications software was first issued in Norway and Sweden in November 2005, and in five additional countries in February 2006.
The latest update has rolled it out to Windows users in America, the UK, Malaysia, Australia and New Zealand.
Redmond is also piloting similar software called Microsoft Office Genuine Advantage which will notify people if their copy of Microsoft Office is genuine.
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