The search engine in Windows Search 4.0 is a Microsoft Windows service that is also used by programs such as Microsoft Office Outlook 2007 and Microsoft Office OneNote 2007.
You can use this search engine to index a program’s content and to obtain instant results when you search in a particular program.
Windows Search 4.0 includes the following improvements:
Support for indexing encrypted documents of local file systems
Reduced affect on Microsoft Exchange when you index e-mail in online mode, and there is no local cache (.ost)
Support for indexing online delegate mailboxes
Support for client-to-client remote query to shared indexed locations
Improved indexing performance
Faster previewer updates for Windows XP
Per-user Group Policy settings
Windows software updates for Watson errors
You can find out more and can downloads updates for XP, Vista SP1 and Windows 2008 from http://support.microsoft.com/?kbid=940157