|You use the built in permissions of Windows XP Professional. |
You can set a folder to be accessible only to named users or members of a particular user group like the administrators group.
You can also use permission so the only designated users or user groups can alter a file and everyone else is read only. You get a lot more flexibility with with in windows XP Prof as you can create your own user groups. XP Home only has three built in user groups.
Look up permissions in the help files. Needs a bit of planning and thought.