The first question you may be saying to yourself is, “I thought the Microsoft download page for Messenger 5.1 said Vista wasn’t a supported OS”.  Well, it isn’t, at least not officially, but it does work (sort of).  The answer to the question above that you don’t want to hear is, “You can’t”.  Windows Vista will not let you install the newest release of Windows Messenger 5.1 on Vista when UAC is installed.  Sure, it will be nice and ask you for permission to install, but even though you say yes it will still fail and say that the “installation has been interrupted.”  The only way you will be able to install Windows Messenger 5.1 on Vista is to actually disable UAC, reboot, install Messenger, and then re-enable UAC, if so desired, and reboot again. 

Now how do you disable UAC? 

1. Click on Start -> Control Panel -> User Accounts and Family Safety -> User Accounts -> Turn User Account Control On or Off.
2. Uncheck the box next to “Use User Account Control (UAC) to help protect your computer”
3. Click on “OK”. 
4. It will say you need to reboot, and it isn’t kidding so go ahead and reboot.
5. Once you’ve rebooted into Vista, install Windows Messenger 5.1 and then follow the same instructions above, this time placing the check in the box, to re-enable UAC.

Be careful though, Windows Messenger 5.1 doesn’t like to work very well when a scrollbar appears in a chat window.  This is somehow tied to the “don’t give out your credit card information” message.  More on that later if I can find an easy answer to how to fix it.