The world's biggest software company, Microsoft, says hackers have broken into its corporate computer network.
The hackers gained access to the "source code", or blueprints, of Microsoft's Windows-based software, which is estimated to run on about 90% of the world's PCs.
But Microsoft's president and chief executive, Steve Ballmer, insisted they had not been able to tamper with any of the company's key programs.
He said: "It is clear that hackers did see some of our source code. "I can assure you that we know there has been no compromise of the integrity of the source code; that it has not been modified or tampered with in any way."