As well as its standard dictionaries, Word keeps a separate one called Custom.dic, which stores all the words you've added to the dictionary during spell checking. If you've inadvertently added incorrectly spelled words and want to remove them, open the Tools menu and click Options. On the Spelling and Grammar tab, click Custom Dictionaries, then tick Custom.dic and select Modify. As well as deleting words, you can enter new ones to build up a bank of specialist vocabulary. To do the same thing in Word 2007, click the Office button (top left), select Word Options, then Proofing, and then click the Custom Dictionaries button.