I'm guessing you are talking about the Word 2007 master source list.
The default sources.xml file is located under
"%appdata%\microsoft\Bibliography\"
If you don't find it there, in Word 2007, you can go to the "References" tab
and click on "Manage Sources". There click on "Browse...". The default
directory where it opens in is the directory where you stored your master
list.
An alternative way of transporting the master source list is through a Word
2007 document. Open a new document. Go to "References", "Manage Sources".
Select all sources from the master list (left list) and use the "Copy ->"
button to copy them to the current list (right). Quit the source manager and
save the document. Then on your fresh install, open the document, go to
"Reference", "Manage Sources". Select all sources in the current list
(right) and use the "<- Copy" button to copy them to the master list (left).
The above method is also useful if you want to merge multiple master lists
(on the same or different computers). The copy function finds doubles during
the merging.
Yves