Messenger Plus! Live is just an add-on for Windows Live Messenger
Downgrading Messenger Plus! Live gives you no advantages, only less features, as Plus only improves features, rarely removing them.
What is your current version of Windows Live Messenger? (Found in 'Help'->'About Messenger')
If you wish, you can find 8.5 (the old version) on
http://www.filehippo.com
Otherwise go to
http://download.live.com to get version 2009.
Either way, you should upgrade to the latest Messenger Plus! Live from
here to ensure you get the full features