This article describes how to remove misspelled words from the custom dictionary in the event that you've added them accidentally, while using the Spell checker in Windows Mail in Windows Vista.
Click Start, type notepad.exe and press ENTER
From the File menu choose Open
Type the file name below and click Open:
%USERPROFILE%\Appdata\Local\Microsoft\Windows Mail\UserDictionary.lex
UserDictionary.lex is a plain-text file which stores the list of words added. Remove the misspelled word that you accidentally added. Make sure that there are no blank lines left by these deletions. Save the file and close Notepad.
%USERPROFILE% represents the path to your profile directory, which is usually located at C:\Users\{username}