Here is a great post on how to resolve an error in Windows Live Messenger when it suddenly stops signing in. 

http://ee61re.wordpress.com/2010/12/14/unable-to-sign-in-to-windows-live-messengererror-81000490/

Delete the C:\users\<username>\AppData\Local\Microsoft\Windows Live\Contacts\<Windows Live ID> folder and then try signing in again.