MSN under Windows XP 64bit.

1) Download the full installer from Microsoft:
http://download.live.com/?sku=messenger

It should be named "wlsetup-all.exe" (not the web version "wlsetup-web.exe), try several times then you will get it.

2) Use this patcher at your own risk :-) I tried it and works..

http://oss.netfarm.it/wlm.php

3) Launch the patched installer

4) Enjoy...
Source(s):
http://www.techspot.com/blog/129/how-to-run-windows-live-messenger-on-windows-xp-x64/

http://oss.netfarm.it/wlm.php