Taken from:

http://mywebpages.comcast.net/SupportCD/OptimizeXP.html

3. Disable Indexing Service
The Indexing Service in Windows XP indexes your files presumably to shorten the time needed to search your hard drive if you are looking for a specific file or part of a phrase inside a file. However, the constant indexing of files actually slows down system performance and does not benefit search performance except for extreme complex searches.

Instructions - To disable the Indexing Service go into "My Computer", right-click on all your hard drive partitions one at a time, left-click "Properties". Uncheck "Allow Indexing Service to index this disk for fast file searching". Select "Apply changes to subfolders and files". If any files cannot be updated select "Ignore All".