Unable to switch computer users

Although all the user accounts are shown in "User Accounts" in the control panel, only one account shows on the startup screen. (Which also has changed) The other accounts are unaccessable when trying to switch users also. Again, only the one account is shown. This was first seen after being away for 5 days. I should have used system restore, but didn't realize the inability to access the other accounts at the time. I am the primary user on this machine and now too much time has passed for system restore. Any ideas?? Could it be an automatic update that caused this, or was someone messing with the settings while I was away? Appreciate any help.
The computer is a Dell 4400...with xp...service pack 3
Thanks, john