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On the Start menu, click Control Panel.
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In Control Panel, open Add or Remove Programs.
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Select Microsoft Visual Studio <version> Professional and then click Change/Remove.
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On the Visual Studio <version> Maintenance screen, enter the product key in the section Upgrade to Visual Studio <version> Professional and then click Upgrade.
For more information, visit this site on Visual Studio 2005: http://msdn2.microsoft.com/en-us/library/ms246600(VS.80).aspx and this site on Visual Studio 2008http://msdn2.microsoft.com/en-us/library/ms246600.aspx