http://support.microsoft.com/?kbid=940157

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