I think your video grphic card does not install properly,its one of cause for that missing switch,Anyway there is some way to "SWITCH USERS"
In Microsoft Windows XP, if you enable the Fast User Switching feature, multiple user accounts can log on to a computer simultaneously. This article describes how to enable and use this feature. With Fast User Switching, users can switch sessions without closing Windows, programs, and so forth. For example, User A is logged on and is browsing the Internet, User B wants to log on to their user account and check their e-mail account. User A can leave their programs running while User B logs on and checks their e-mail account. User A can then return to their session where their programs would still be running.
Fast User Switching is enabled by default in Windows XP Home Edition and Professional on computers with more than 64 megabytes (MB) of RAM. However, Fast User Switching is not available on Windows XP Professional-based computers that are part of a domain network.
Enable or Disable Fast User Switching
Note: To enable Fast User Switching, you must also enable the Use the Welcome screen option. This feature cannot be used if your computer is a member of a domain. To enable Fast User Switching: 1.Log on to the computer as an administrator.
2.Click Start, click Control Panel, and then click User Accounts.
3.Click Change the way users log on or off.
4.If it is not already selected, click to select the Use the Welcome screen check box.
NOTE: The Use Fast User Switching check box is unavailable until you click to select the Use the Welcome screen check box.
5.Click to select the Use Fast User Switching check box, and then click Apply Options. Quit the User Accounts tool.
Switch Users From the Task Manager
When a user initiates the Switch User option, the computer returns to the Welcome screen. The current user's session remains active and another user can now log on and use the computer. You can initiate the Switch User command in any of the following ways: •Click Start, click Log off, and then click Switch User.
•Open Task Manager (CTRL+ALT+DELETE), and then click Switch User on the Shut Down menu.
•Hold down the Windows key, and then press the L key).
Switch Users From the Standard Logon Screen1.Click Start, click Log Off, and then click Switch User. The Windows XP logon screen appears and displays the number of running programs for each user under that user name.
2.Click the user that you want to switch to.
3.Type your password, and then click the arrow button to log on to the computer.
Manage Other User Sessions
With administrative privileges you can log other users off from the computer, which frees up system resources, and you can send them a message from your session. To do this: 1.Open Task Manager (CTRL+ALT+DELETE).
2.On the Users tab, click the name of the user that you would like to log off or send a message to.
3.Click Logoff to log the user off from the computer, or click Send Message to send the user a message.
Troubleshooting
Fast User Switching cannot be used when Offline Files are enabled. When you attempt to enable Fast User Switching, you may receive the following error message when you click Change the way users log on or off:
Fast User Switching cannot be used because Offline Files is currently enabled.
To make changes to Offline Files, click OK.
To resolve this issue, disable Offline Files: 1.When you receive the preceding error message, click OK. The Offline Files Settings dialog box appears.
2.Click to clear the Enable Offline Files check box, and then click OK.
NOTE: You will no longer be able to work with files and programs that are stored on the network when you are not connected to the network.
Your computer may run slower than you expect. If you leave many programs running when you switch users, you may notice that your computer runs slower than usual. To resolve this issue, quit some programs before you switch users.
Other users can turn off the computer while programs remain open. When you switch users, another user can turn off the computer. If a logged on user has programs running, they will be closed when the computer is shut down, and this may result in data loss. To prevent this, save your work before you switch users.