Windows Messenger was an instant messaging client from Microsoft which could connect to Microsoft .NET Passport (now Windows Live ID), SIP (session initiation protocol) communication services, and Microsoft Exchange Instant Messaging. Windows Messenger 5.1 was included with Windows XP but was not released for any newer versions of Windows (it has been replaced by newer products like Windows Live Messenger and Office Communicator). You can obtain more information about Windows Messenger from the Windows Messenger How-To Center on Microsoft’s web site.
Windows Messenger is fairly out-dated and does not have many of the features included in newer versions of MSN Messenger/Windows Live Messenger (see this Microsoft document for some details). It can still can be useful (or even necessary) for some things. It is especially handy because it works alongside MSN Messenger/Windows Live Messenger and is not affected by their installations. If Windows Live Messenger is broken or unable to connect you can use Windows Messenger as a backup. Windows Live Messenger also cannot connect to SIP communication services or Exchange instant messaging.
You can download the installer for Windows Messenger 5.1.0701 from here.
When you run the “Messenger.msi” installation file under Windows Vista or Windows 7 it fails partway through the installation process, reporting that the installation was interrupted. The problem is caused by a compatibility issue between the MSI package and UAC. You cannot simply adjust compatibility modes or elevated privileges for standalone MSI packages like you can for setup executables because they are handled by the Windows Installer service.
To get it to install properly, open an elevated command prompt (right click the “Command Prompt” shortcut under the start menu and select “Run as Administrator”). From the administrative command prompt enter the commands:
msiexec /a <path>
msiexec /i <path>
Where <path> is the location of the messenger.msi file. So if I saved it to my desktop, the commands would be:
msiexec /a C:\Users\Keith\Desktop\Messenger.msi
msiexec /i C:\Users\Keith\Desktop\Messenger.msi
The installation will run and should be able to complete successfully.
Posted by Keith Balomben 
Keith B's Tech Blog 