NOTE: This post is old and not relevant anymore. I have removed the link as it no longer works!
Facebook has just released its new messenger for windows but it only runs on Windows 7.
Prior to this there was a leaked version whilst in beta phase that ran on Windows Vista.
The new version may also run on Windows Vista but I haven't bothered to try it out by downloading on a Windows 7 computer and installing it on another, I do know however that this version runs fine and there is no difference that I see from the latest version.
So if you would like to run Facebook Messenger on Windows Vista then download it from this link.