Securing public computers isn't easy, but a Microsoft kit to do that may also help families share home computers. If you share a Windows XP Home or Professional computer with other users, particularly, children, or family members, you may find the Shared Computer Toolkit beta an interesting idea. But caveat emptor: this is free "beta" software, has a spooky "validation" process to confirm your copy of Windows is "genuine," and installation is not for the faint of heart. Then again, neither is re-installing Windows after a friend/relative/SO inadvertently trashes your computer.
http://www.microsoft.com/windowsxp/sharedaccess/default.mspx