This issue dealt with an application install failure on a 64-bit installation of  Windows Vista.  The application in question required Windows Vista Service Pack 1 as a prerequisite for installation, and the system administrator duly installed SP1 and then proceeded to launch the installation of the application.  And then, a funny thing happened – the application returned an error, indicating that Service Pack 1 for Windows Vista was not installed.  According to the Windows Update logs, the SP1 installation appeared to have completed without incident.  Trying the installation of the application on another machine, the administrator found that he encountered the same problem on every Windows Vista SP1 machine.  As Alice said, while in Wonderland, “curiouser and curiouser” …

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Additional Resources:

• MSDN: GetVersionEx function