Microsoft Office 2007 applications (at least Outlook and OneNote in particular) require Windows Desktop Search 3.0 to be installed for their built-in search capabilities to work fully. Since out of the box this version of WDS doesn’t support the indexing of files on network shares you also have to install the WDS Add-in for Files on Microsoft Networks. You still have to map the network drives to be indexed but at least you can select them in the WDS options window afterwards.
In Windows Vista WDS 3.0 is already built-in but you still have to install the add-in for the indexing of files on network shares to work.