I know this question has been asked on other forums, and it is also not the easiest problem to diagnose, but Windows Live Messenger has never worked on my Vista x64 rig. I get the "Windows Live Communications Platform has stopped working" message when signing in.  I installed and ran windows live 2k9 as one of the first things out of the box, I installed x64 on my commuter myself so I'm not sure if i might be a driver problem or not. I do not have any custom windows themes.  If you can solve this than you will be my hero.  (Think about how AWESOME that is)  If you need anymore info than i can provide. (sorry if that was choppy or hard to understand??)

You could try looking at some of the possible solutions in similar threads such as: Windows Live Communications Platform Error! and -SOLVED- Windows Live Communications Platform has encountered ........

dollar's reply to -SOLVED- Windows Live Communications Platform has encountered ....... seems to work.

Note that a lot of so called 'fixes' will actually cripple stuff more, eventhough some things might seem to work at first.

(yes, there are some problems apparently with the upgrade, but many of those problems arise from (wrong) stuff people did in the past to 'fix' some other issues)

So, please ALWAYS search the official forums of Messenger Support FIRST when you want something fixed, before you attempt to do anything else:
http://messenger-support.spaces.live.com/blog/cns...8B853F!15941.entry
(There are more official sites than this one though. For example http://help.live.com/)

And if the given solutions don't work, then you can always contact Live Support by mail:
http://support.live.com/eform.aspx?productKey=wlm...&ct=eformts&scrx=1

This will not only fix your issue for sure, but will also tell Microsoft about the problem so they can permanently fix it in next updates. And that is exactly the major big issue with all those so called 'fixes' on the net: they do not give Microsoft the chance to investigate and fix the bugs properly.