Because it doesn't work on E-messenger to... maybe you're connecting via proxy? Make sure you directly access the .NET service. If you aren't, changing the setting might give a difference. If it doesn't work; try to sign in on a friend's computer. If you are able to sign in there, it is your pc doing strange. And if you have no clue what the cause is of the problem; you might do a restore point of a previous date when it was still working, else you re-install msn-messenger properly, if it still doesn't work, re-install windows, when still no result, I would backup my important files and format the harddisk....
Succes
Edit: I didn't read far enough, i now see the problem is already solved
greetz