What's the Windows Installer rollback functionality?

A. When you install a Windows Installer (.msi) file, a script in the file specifies the actions that Windows Installer will perform. As each action is performed, the process that calls the Windows Installer file into action updates a rollback script and, if files are to be deleted, backs up those files. Then, if the installation fails, Windows Installer can use the rollback script to undo any actions that have been performed and use the file backups to restore any deleted files. After the installation completes successfully or is rolled back, Windows Installer deletes the file backups because they're no longer needed.

You can define custom actions to be performed during a Windows Installer file installation. For example, you can

Windows Installer doesn't roll back these custom actions by default. Rather, the author of the Windows Installer file is responsible for ensuring that custom actions are rolled back. By default, no additional action is needed to roll back an installation that fails.

The files that Windows Installer temporarily backs up are stored in the %systemdrive%config.msi file in a system/hidden state. However, be aware that users can access these files and perhaps gain information about your system that you might otherwise prefer to restrict.