After installing Microsoft Office Outlook 2007 on Windows XP you may receive an informational dialog box when starting Outlook that says: "Windows Desktop Search is not currently installed or not up to date. Outlook will not be able to provide fast search results when using the new Instant Search functionality unless this Windows component is installed."
The Instant Search functionality of Outlook 2007 requires a component called Windows Desktop Search (WDS) to be installed. Without this you still have all the same search functionality, but results won't be returned as quickly. You can check the box that says not to show the message again--and have slower search results--or you can install WDS.
To install WDS version 3, download from the URL below, log on as administrator and install. It will take some time for the initial index to be built, but thereafter your search results should be noticably faster.