Can't change key in Visual Studio 2008 installer

I have a trial version of Visual Studio 2008 Professional Edition but also a genuine serial key from Technet. When I install the application, I cannot change the predefined key. How can I then use the unlimited version (what m开发者_开发知识库y key represents)?

Thanks

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You can't. You will probably have to uninstall the trial version and install from your Technet media.