Edit: Zahid beat me to it
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1. Rename your MSN Messenger folder, usually located in C:Program Files, to something else, e.g. MSN Messenger 7.5
2. Create a new shortcut to the msnmsgr.exe in that folder (just right-click msnmsgr.exe and click Send to Desktop (shortcut)).
3. Install Windows Live Messenger
To run both of them simultaneously you will need to patch them with polygamy, you can do that with
A-Patch.