I suggest you do a completely fresh install (yeah, this means loosing your custom content aswell):
1. Follow all instructions (point 1 to 9) provided in
Completely remove (MSN/Windows Live) Messenger
2. Redownload and install the latest WLM (8.1.0178) at
http://get.live.com/messenger/overview
3. Redownload and install latest Plus!Live from
http://www.msgpluslive.net/download/