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When trying to install ArcGISArcReader90sp1.msp, the following error message from Windows Installer is returned:
"The upgrade patch cannot be installed by the Windows Installer Service because the program to be upgraded may be missing, or the upgrade patch may update a different version of the program. Verify that the program to be upgraded exists on your computer and that you have the correct upgrade patch."
The ArcGIS Desktop 9.0 SP1 is installed. The Service Pack for ArcReader does not need to be installed. The patch is only necessary for ArcReader standalone on systems that do not have ArcGIS products installed.
Do not attempt to install the ArcReader patch if ArcGIS 9.0 SP1 is already installed.