this is for windows xp, not sure how the task manager works in any other os, if some one could post how to do in another os then great.
do ctrl + alt + delete.
on the process tab, look for msmsgs.exe click on it and then click end process, do the same for msnmsgr.exe and MessengerDiscovery, in this order.

now go to the control panel and go to the add or remove programs. then find MessengerDiscovery, and uninstall it.

start up msn messenger and you shouldnt get any signing in problems, and the program should be removed.
Hope this helps.

You might need to disable the proxy as well (Tools > Options > Connection).

You can use the command !forceexit to close MessengerDiscovery .

Also on uninstall all proxy settings are returned so that the user should never have to mess with the proxy settings .