The Microsoft Windows Installer is an 'application installation and configuration service'. This means that it is used for installing programs on your computer. Some software requires this latest download before you can install it. In effect, this becomes a two step process. First you download the Windows Installer and run it (restart your computer). Then run your software's installation program (yes, there is some degree of Microsoft kookiness that requires you to install an installation program before you can install the program you actually want).
According to Microsoft, this is the function of the package: 'It 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.'
Microsoft makes the following note for users of previous versions of the Windows Installer: 'Windows Installer 3 file follows a new naming convention that contains a knowledge base reference so it is easier to identify which version of the Windows Installer will be installed. The previous naming convention, INSTMSI.EXE, will no longer be used.'
Bottom line: if you try to install a new program on your computer and it says it needs Windows Installer, then download this package. This 4.5 release fixes numerous bugs, so it may be worthwhile downloading if you are experiencing installation problems.