In other instances, the iTunes installation fails completely with error 1603 (which is generic 'fatal error' code). Just some of the symptoms I've noticed in my environment. In Event Viewer's Application log, you may see Event ID 10005 from Source MsiInstaller, stating the dreaded error message 'A later version of Apple Application Support is already installed on this computer.' In Programs & Features, there is no Apple Application Support to uninstall. In Event Viewer's Application log, you may see Event ID 1000 from Source Application Error, stating 'Faulting application name: MsiExec.exe' along with a bunch of other error details. When attempting to start iTunes, you may receive popup error 'Apple Application Support was not found.

Apple Application Support is required to run iTunes. Installing Apple Application Support via the extracted MSI (AppleApplicationSupport.msi) fails with the same error: ' A later version of Apple Application Support is already installed on this computer.' Uninstalling and installing iTunes fails, including via the steps described for a 'full uninstall' as described here on the Apple site (for instance: Troubleshooting issues with iTunes for Windows updates) Please uninstall iTunes, then install iTunes again'.

(now that I know the issue, I'm confident any previous version of the installer will fail). If you run the MSI installation with verbose logging, you may see lines stating: 'PROPERTY CHANGE: Adding BNEWERPRODUCTISINSTALLED property.

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