You could always try deleting the MSN Messenger Application Data From: C:\Documents and Settings\<win Username>\Application Data\Microsoft\MSN Messenger. you'll lose any custom display pics/emotions etc tho.
Once your in that folder download the id number Identifier (from here:
http://shoutbox.menthix.net/showthread.php?tid=15519&page=&#pid=132217 (see 3rd post) then work out your number and delete the folder.