The Redmond, Wash.-based software firm is still investigating the apparent theft of the software, she said.

It is believed a copy of the OS, which is being tested internally by the company, came directly from Microsoft´s internal Web site. It is not unusual for software in Microsoft´s public beta testing program to show up on the Web, but it is unusual for software that is still being tested internally, known as internal builds, to show up there.

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