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Building up a solid custom dictionary in Microsoft Word takes a while as you add a handful of words at a time, but once you get it built up, it makes working on your documents a much more pleasant task.
So what do you do when you need to re-install Windows and do not want to lose your custom dictionary?
When you add a word to the dictionary in Word 2010, it appears in a text file here:
C:\Users\%username%\AppData\Roaming\Microsoft\UProof\CUSTOM.DIC
You can copy this file over to your new installation.
As an added Note:
The user name might not match the actual folder name, and Windows could be installed in a different drive. You can use this instead:
%AppData%\Microsoft\UProof\CUSTOM.DIC
The next time you have to re-install Windows, you will be able to save and re-use your custom dictionary with ease now that you know where to look!