I've been having the same problem as a number of people where the nick is not picked up correctly due to server changes.
This is a fairly basic plugin that offers the suggested functionality of setting the nick each time you login.
This is my first amsn plugin so it would be nice to have someone check it. The only testing has been with 0.98.1 on linux, I would imagine that required version number can probably be lowered but I don't have old versions installed to test with and I see no reasons why it shouldn't work on all platforms.
Instructions1) Download from:
http://www.mediafire.com/file/hz3xwlcmzt2/autonick.zip2) Unzip in plugins directory
3) Load the plugin
4) Click configure and enter the nick you want
Sorry for the dupe if this has already been done but I could only see discussion and no actual plugin.