I found this in a few different places around the 'net - hopefully it's helpful:

Turn Off Indexing to Speed Up XP

Windows XP keeps a record of all files on the hard disk so when you do a search on the hard drive it is faster. There is a downside to this and because the computer has to index all files, it will slow down normal file commands like open, close, etc. If you do not do a whole lot of searches on your hard drive then you may want to turn this feature off:

• Open My Computer.
• Right-click your hard drive icon and select Properties.
• At the bottom of the window you'll see "Allow indexing service to index this disk for faster searches," uncheck this and click ok.
• A new window will pop up and select Apply to all folders and subfolders.

It will take a minute or two for the changes to take affect but then you should enjoy slightly faster performance.