Microsoft Knowledge Base Article 907749 contains the following summary:

This article contains an overview of Microsoft Windows Installer and how to troubleshoot issues that may occur when Windows Installer is installed.

You can use Microsoft Windows Installer to install and to configure installation packages. An installation package contains all the information that Windows Installer requires to install or to remove an application or a product and to run the setup user interface. Each installation package includes an .msi file that contains an installation database, a summary information stream, and data streams for various parts of the installation. The .msi file can also contain one or more transforms, internal source files, and external source files or cabinet files that are required by the installation.

The following is a summary of the available versions of Windows Installer:

Windows Installer 3.1: Windows Installer 3.1 is included in Microsoft Windows Server 2003 Service Pack 1 (SP1). Windows Installer 3.1 is also available as a redistributable system component for Windows 2000 SP3, Windows 2000 SP4, Windows XP, Windows XP SP1, Windows XP SP2, and Windows Server 2003. For more information about Windows Installer 3.1, visit the following Microsoft Web site:

http://www.microsoft.com/downloads/details.aspx?FamilyID=889482fc-5f56-4a38-b838-de776fd4138c

To install Windows Installer 3.1 as a critical update, visit the following Microsoft Web site:

http://windowsupdate.microsoft.com

Windows Installer 3.0: Windows Installer 3.0 is included in Windows XP SP2. Windows Installer 3.0 is also available as a redistributable system component for Windows 2000 SP3, Windows 2000 SP4, Windows XP, Windows XP SP1, and Windows Server 2003. For more information about Windows Installer 3.0, visit the following Microsoft Web site:

http://www.microsoft.com/downloads/details.aspx?FamilyID=5fbc5470-b259-4733-a914-a956122e08e8

Windows Installer 2.0 : Windows Installer 2.0 is included in Windows Server 2003, Windows XP, and Windows 2000 SP3. Windows Installer 2.0 is available as two packages. One package is used to install or to upgrade to Windows Installer 2.0 on Windows Millennium Edition, Windows 98, and Windows 95. The other package is used to install or to upgrade to Windows Installer 2.0 on Windows 2000 and Windows NT 4.0 SP6. For more information about Windows Installer 2.0 for Windows Millennium Edition, Windows 98, and Windows 95, visit the following Microsoft Web site:

http://www.microsoft.com/downloads/details.aspx?FamilyID=cebbacd8-c094-4255-b702-de3bb768148f

For more information about Windows Installer 2.0 for Windows 2000 and Windows NT 4.0, visit the following Microsoft Web site:

http://www.microsoft.com/downloads/details.aspx?FamilyID=4b6140f9-2d36-4977-8fa1-6f8a0f5dca8f

Note Do not use these packages to install or to upgrade Windows Installer 2.0 in Windows Server 2003 or in Windows XP.

Windows Installer 1.2: Windows Installer 1.2 is included in Windows Millennium Edition. Windows Installer 1.2 is also supported on Windows 95, Windows 98, and Windows NT 4.0 SP3 or a later version of Windows NT 4.0.

Windows Installer 1.1: Windows Installer 1.1 is included in Windows 2000. Windows Installer 1.1 is available as a redistributable system component for Windows 95, Windows 98, and Windows NT 4.0 SP3 or a later version of Windows NT 4.0. For more information about Windows Installer 1.1 for Windows 95 and Windows 98, visit the following Microsoft Web site:

http://www.microsoft.com/downloads/details.aspx?FamilyID=f1b7a5b0-611b-45a7-a06d-cadff33e64da

For more information about Windows Installer 1.1 for Windows NT 4.0, visit the following Microsoft Web site:

http://www.microsoft.com/downloads/details.aspx?familyid=F8CA7781-41DC-4A53-A574-82EB9CD85BBB

Windows Installer 1.0 : Windows Installer 1.0 is included with Microsoft Office 2000. Windows Installer 1.1 is also supported on Windows 95, Windows 98, and Windows NT 4.0 SP3 or a later version of Windows NT 4.0.
To determine which version of Windows Installer is installed on a computer, follow these steps:
1. Click Start, click Run, type %systemroot%\system32, and then click OK.
2. Right-click Msi.dll, and then click Properties .
3. Click the Version tab, and then note the File version number.