Windows Search for Windows XP helps you to find, preview, and use your documents, e-mail, music, photos, and other items.
It is a required installation when you want to make use of the Instant Search (indexed search) feature of Outlook 2007 and Outlook 2010 on Windows XP.
Windows Search has been integrated in Windows Vista and later so you won’t need to install it anymore.
Download: Windows Search 4.0 for Windows XP
In case you are running into issues with Instant Search in Outlook see: Fixing Instant Search