I noticed recently that the Pidgin 2.7.5 messenger [1] had problems connecting to MSN with a certificate error about omega.contacts.msn.com. Fortunately I’m neither the first to have that problem, nor to find a solution [2].
MSN apparently replaced their certificate, which now needs to be updated with Pidgin.
1. point your browser to https://omega.contacts.msn.com/
2. get the certificate and export to a file
3. name the file omega.contacts.msn.com
4. copy the file to %AppData%\purple\certificates\x509\tls_peers and replace
5. restart pidgin
[1] http://www.pidgin.im/
[2] http://intertwingly.net/blog/2010/11/18/Reconnecting-with-omega-contacts-msn-com
George's Techblog 