Did you download and install the latest windows updates within the past few days? A lot of people seem to be having this problem with WLM after having done that.
Close Messenger completely before trying this:
1. Hit "run" in the start menu
2. Type "regedit"
3. Open "HKEY_CURRENT_USER\Software\Microsoft\MSNMessenger"
4. Delete the folder called MSN Messenger
After this MSN Messenger Live will login as normal.
or...
If you're not comfortable editing the registry, you can run this small reg. file posted by
Jonathan Kay [MVP] which does the same thing as posted above:
Remove Messenger Settings