From Microsoft:Windows Installer installs and configures your products and applications reducing the total cost of ownership.

What Do You Need To Know About Free Software?

The Microsoft Windows Installer is an installation and configuration service. Windows Installer enables software users to efficiently install and configure products and applications. The installer can also provide software products with new capabilities to advertise features without installing them, to install products on demand and to add user customizations. The Windows Installer supports advertisement of applications and features according to the operating system.

Version 4.5 of the Windows Installer has several new features, including: Custom Action Execution On Update Uninstall–This feature enables an update to add or change a custom action so that the custom action is called when an update is uninstalled. Shared Component Patching Resiliency During Uninstall–This feature makes sure that the most recent version of a component is available to all the products.