The Windows Indexing Service is always running in the backround on your PC and can slow your hard drive and cpu down a bit. The Indexing Service creates indexes of the contents and properties of all files on local and network drives in order to increase file searching speed. Turning this off will make your hard drive and cpu run faster at the cost of a slower search when you do a windows search. I don't know about you, but I don't think I do more than a few windows file searches per year. I also use google desktop which has its own indexing service. Having both of them run constantly can definately slow your computer down
To do disable windows indexing, open up "My Computer", Right Click on C:, Click properties, Uncheck "Allow Indexing Service to index this disk for fast file searching", click ok.