Try this, it worked for me! FINALLY!! :D :D
If Messenger doesn’t remember your e-mail address or your password, then you have installed both Windows Live Messenger and Yahoo Messenger. You have to revert a registry setting that has been changed while Yahoo Messenger was installing. Then you will be able to store your sign-in credentials again. Here is how it works:
Open the registry.
Windows XP: Click Start, Run. Enter regedit, then click OK.
Windows Vista: Click Start, type regedit into the search box, then press Enter.
Now open the folder User Shell Folders. Use the following path: HKEY_CURRENT_USER/Software/Microsoft/Windows/CurrentVersion/Explorer/User Shell Folders.
Now, a list with several entries appears on the right side. Double-click AppData.
At Value, enter the following and click OK:
Windows XP: %USERPROFILE%\Application Data
Windows Vista: %USERPROFILE%\AppData\Roaming
In Windows Vista, do the same for the entry Local AppData with its value %USERPROFILE%\AppData\Local.
Close Windows Registry Editor, and reboot your computer.
Source:
http://messengermaximal.spaces.live....814!8064.entry