If you have Windows XP, it is called "custom.dic" and is located at
C:\Documents and Settings\[user profile]\Application
Data\Microsoft\Proof\custom.dic
If you have Windows 95/98/Me, it is still called "custom.dic" and it is
something like C:\WINDOWS\Application Data\Microsoft\Proof\custom.dic
I am not entirely sure on the Win9x path. Search WINDOWS\Application Data
for custom.dic. Make sure your search includes subfolders. It is nested
somewhere in that directory.