Vista comes with a desktop indexing service that is developed to make Window's search function faster and easier. The Windows index contains information on the most common files on your computer, including your personal files (e.g. Documents, Music, and Pictures). When you perform a search, Windows will scan this index first, rather than look through the entire hard disk for it. This shortens the search time to a fraction of what it would otherwise be.
By default, the indexing service will constantly scan your files and folders, indexing them so that they will be readily available when you need to do a quick search. It runs quietly in the background, accessing your hard disc and system resources.
Unlike most other desktop search tools, this service does not wait until your PC is idle to index your hard drive. It will be active even as you work on other tasks, significantly lowering system performance. What's more, the more files you have, and the more these files change, the heavier the load is placed in the indexing service. By extension, the indexing service will be more demanding of system resources, which slows down your system.