Microsoft Knowledge Base Article 884016 contains the following introduction:

Microsoft Windows Installer 3.0 shipped as part of Microsoft Windows XP Service Pack 2 (SP2) and is now available as a redistributable system component for Microsoft Windows 2000 SP3, Microsoft Windows 2000 SP4, Microsoft Windows XP, Microsoft Windows XP SP1, and Microsoft Windows Server 2003. The additional features help make servicing easier and more efficient. (Servicing includes authoring, creating, distributing, and managing updates to applications.) These features also address issues that exist with earlier versions of the Windows Installer.