problems with fast user switching

For some reason my WinXP Pro system recently disabled Fast User Switching. When I tried to re-enable it, I was first told I needed to disable offline files, which was quite a surprise because I never consciously enabled them. After disabling offline files,
the control panel lets me enable Fast User Switching, but everytime I try to switch between users I get the Unlock Computer Dialog saying that the computer has been locked by (current user) and I need to unlock it. It asks for user name and password. Sinc
e none of the accounts on this box have passwords, I just hit return and it kicks me back into my user account.

The only way to successfully switch users is to log off of one and onto another. Also, I've set the Welcome screen interface to be ON, but it still gives me the old style logon window with the input fields for user name and password.

Anyone know why it keeps thinking the computer is locked when I try to switch users? Any help would be way appreciated..