The Microsoft Windows Installer is an application installation and configuration service. Instmsi.exe is the redistributable package for installing or upgrading Windows Installer.
Windows Installer is a component of the Windows 2000 operating system and that simplifies the application installation process.
Windows Installer manages the installation and removal of applications by applying a set of centrally defined setup rules during the installation process. These setup rules define the installation and configuration of the installed application. In addition, you use this service to modify, repair, or remove an existing application. The Windows Installer technology consists of the Windows Installer service for the Windows operating systems and the package (.msi) file format used to hold information regarding the application setup and installations.
Windows Installer is not only an installation program; it is also an extensible software management system. Windows Installer manages the installation, addition, and deletion of software components, monitors file resiliency, and maintains basic disaster recovery by way of rollbacks. Additionally, Windows Installer supports installing and running software from multiple sources, and can be customized by developers that want to install custom applications.
With Windows Installer and the .msi package file format, software installation and removal has become more reliable and resilient while providing a larger set of installation options. Windows Installer performs the following tasks:
* Restores original computer state upon installation failure: Windows Installer keeps track of all changes made to the system during the application installation process. If the installation fails, Windows Installer can restore, or roll back, the system to its initial state.
* Helps prevent certain forms of inter-application conflicts: Windows Installer enforces installation rules that help to prevent conflicts with shared resources between existing applications. Such conflicts can be caused when an install operation makes updates to a dynamic link library (.dll) shared by an existing application, or when an operation deletes a dynamic link library shared by another application.
* Reliably removes existing programs: Windows Installer can reliably uninstall any program it previously installed. It removes all the associated registry entries and application files, except for those shared by other installed software. You can uninstall an application at any time after a successful installation. (Removal should not be confused with rollback, which restores a computer to its initial state when an installation failure has occurred.)
* Diagnoses and repairs corrupted applications: An application can query Windows Installer to determine whether an installed application has missing or corrupted files. If any are detected, Windows Installer repairs the application by recopying only those files found to be missing or corrupted.
* Supports on-demand installation of application features: Windows Installer can be instructed to initially install a minimal subset of an application. Later, additional components can be automatically installed the first time the user accesses features that require those components. This is known as advertising. For example, Windows Installer could install Microsoft Word with a minimal set of features. The first time the user tried to access a mail merge function (not included with the original installation), Windows Installer would automatically install the mail merge component. Similarly, Windows Installer can also purge components that go unused in an application.
* Supports unattended application installation.