You can change that option for Microsoft Office Word 2007 by following these steps but I couldn't find how to change it in other Office 2007 applications.
1 - Open Microsoft Office Word 2007
2 - Click the Office button and select Word Options
3 - Select Advanced tab
4 - Scroll down to General and click File Locations button
5 - Select Documents and click Modify button
6 - Navigate to the folder that you want to use as the new default open/save folder
7 - Click OK 3 times to close all dialog boxes
For other Office applications you may try adding that particular subdirectory to "My Places" bar which appears in the left pane of the Open/ Save As dialog boxes. That makes it much easier to navigate to your favourite places. Just follow the steps in that KB article from Microsoft Help and Support: http://support.microsoft.com/default.aspx/kb/826214/en-us