Usually the reasons you get that 'unable to sign you in' is due to bad password or bad username. If your certain its correct,the next step I would do is eliminate the suspect software form the equation. Try an alternative to MSN. One simple way is to test your account with the dmsn.
http://www.dmsn.nl/
Also, there are many other alternatives you can try to access your account, without using the Offical Messenger client from MS.
http://www.oldversion.com/talk/index.php?a...t=ST&f=2&t=1395