Thought I’d drop a line and remind you that we’re planning to release Windows Search 4.0 (KB940157) via Windows Update sometime in late July. Windows Search 4.0 updates previous versions of Search in Windows Vista SP1, Windows XP SP2 and Windows XP SP3, Windows Server 2003 SP2, Windows Server 2008, and Windows Home Server.
Windows XP users will see Windows Search 4.0 available as an Optional update, which means you must visit the Windows Update Web site to install it. Windows Vista users with SP1 installed will see the update as a Recommended update. This means that, if a Windows Vista user has set their machine to automatically install updates, Windows Search 4.0 will automatically install on PCs running Windows Vista SP1. The release of Windows Search 4.0 to Windows Update will be gradual for these users. If you’re running Windows Vista SP1, and you would like to install it right away, you can go to the Windows Update control panel and install it manually once it is made available.
Source- Technet Blog