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:

1. 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.
2. Now open the folder User Shell Folders. Use the following path: HKEY_CURRENT_USER/Software/Microsoft/Windows/CurrentVersion/Explorer/User Shell Folders.
3. Now, a list with several entries appears on the right side. Double-click AppData.
4. At Value, enter the following and click OK:
   • Windows XP: %USERPROFILE%\Application Data
   • Windows Vista: %USERPROFILE%\AppData\Roaming
5. In Windows Vista, do the same for the entry Local AppData with its value %USERPROFILE%\AppData\Local.
6. Close Windows Registry Editor, and reboot your computer.

Source: http://messengermaximal.spaces.live.com/blog/cns!40F43EC9BA81D814!8064.entry

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