James,
What are the specs of your Windows XP machines? Do they meet the minimum requriements outlined in the guide that Tim mentioned (i.e. http://www.ptc.com/WCMS/files/125512/en/Windchill10xClientRequirements.pdf).
Are your users on Windows XP running on a WAN or a LAN?
What exactly is slow (i.e. general modeling within Creo or workspace actions, such as check out and check in)?
Are the workspace caches local to the machines?
Is the Creo install local to the machine?
Are you using the same config.pro files across all clients?
If you delete/rename and recreate the local workspace cache is there any improvement?