There have been ongoing problems with MSN Messenger since around Jan 2002 with alot of speculation about the problem but no real solutions that I have seen. Some believe the problem lies with any .net passports vs msn passports. Some believe the problem is with LinkSys routers (although others with the same routers have no problems). Some believe the problem occurs when MS messenger services internally pass users from one server to another. Whatever the problem may be, MS seems to deny that a problem exists.
A solution: uninstall MSN Messenger and install Trillian. This has helped some users but not others. The only downside is that Trillian does not support voice/video so if you use/need those Trillian won't help and you'll have to wait out a solution.
Sorry:>(
DrBob:>)