user switching disables itself

Fast user switching used to work on our XP Home system, but now it does not. When logging off, the button appears, but nothing happens when I click. 'Start-L' locks the computer. I check the setting for user accounts, but each time I go back it is turned off again. I checked the offline files stuff and it's fine.

There was a similar thread with no conclusion:
http://techsupportforum.com/showthread.php?t=56719

Thanks in advance.