You might have added lot of words to the default dictionary when you working in MS-Word. You can take backup of this dictionary if you are going to use another system, or going to format the present system.
The dictionary will be at "C:\Documents and Settings\UserName\Application Data\Microsoft\Proof" with the name CUSTOM.DIC
Where C:\ is your system drive, and Username is your login name.
Now you can put this file in another machine at the corresponding location, your word will start using this file as its dictionary.