Why does Windows XP slow down when I enable fast-user switching?

A. When you use an XP system in a workgroup, the fast-user switching feature lets multiple users log on to the same computer without requiring the current user to close his or her programs. So, the programs that the first user is running when the next user logs on are still running and using system resources. As a result, the machine tends to slow down just as it would if you were running those same applications in the current session.

This slow performance isn't a bug, and users need to be aware that the computer is still running the programs in the switched session. To improve performance, you must log on to the original session and close some of the programs. If logging on and closing down some of these other programs isn't an option, you can restart the computer to close the other sessions and free resources. If you're an administrator, you can run Task Manager and terminate other users' processes.