Can't switch users in XP without logging off

Solution 1:

You have to have the fast user switching feature enabled for this to work. Maybe something disabled it. You can find more information in the article on enabling 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. Click to select the Use Fast User Switching check box, and then click Apply Options. Quit the User Accounts tool.