From Vistas "help" section:
"What files are indexed?
By default, all of the most common files on your computer are indexed. Indexed locations include all of the files in your personal folder (such as Documents, Pictures, Music, and Videos), as well as e‑mail and offline files. Files that are not indexed include program files and system files.
The locations that contain those files are not indexed because you rarely need to search those files, and so not including them in the index makes your searches perform faster." [/b]