Hi,
You have to Rearm it then enter the new key.

>Less then a month ago, the IT department gave me a newly formatted computer with Windows 7 installed. Over the last weekend, the machine decided that the version of windows OS is not genuine. Has anyone else experianced this, and what causes it, and how to fix it.
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>This has happened on a older machine, and my machine at home. Research on the web has proved nothing that resolves the issue. I have tried to re-register it, but the tool does not launch because the OS is not genuine.