If it's been a real long time, it's very possible that msn closed the account, and then someone re-opened it. I know for sure that if you don't log in for a long time, MSN will deactivate the account, and then you have to manually re-activate. Also, if you look into the
Msn Messenger Protocol, a little crafty programming can allow you to log in with an account you already have, and in some parts of the protocol, Masquerade as another user. In this example:
MSG bob@passport.com Bob 89\r\n
MIME-Version: 1.0\r\n
Content-Type: text/x-msmsgscontrol\r\n
TypingUser: bob@passport.com\r\n\r\n
Which is also found in the above linked reference, it would be really easy to change the parameter in the MSG command, or the typinguser command. I do a lot of network programming (with sockets), and this is not only possible, but feasible. While I'm sure this is a rarity, and most likely (in my opinion) not the culprit of your situation, it is very possible. I think that the account expired, and was re-opened with someone else....