Apart from the feature work that we did in Windows Installer 4.5, we made a few changes to Windows Installer to address some user feedback or pain points. Here's some of the important issues that were addressed:

1. Added SeBackupPrivilige back to the Windows Installer service. This sould help any custom actions that needed this privilige like the ones that were reported on the Vista Compatability Team blog.
2. Some case sensitive service name comparisions in InstallValidate used to result in an unnecessary files-in-use message on Vista. This is now fixed in Windows Installer 4.5.
3. When a patch added new content in the form of a new component and that patch was being uninstalled, we used to remove that content, even if that content is shared by other products. This is now fixed in Windows Installer 4.5.

In addition to this, since Windows Installer 4.5 is the latest release of Windows Installer, it will have all the fixes and feature work that we did till Vista SP1.

[Author: Hemchander  Sannidhanam]
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