Cannot log on to User Accounts

Although there are three user accounts on my computer, I
cannot log in to two of them...each time I try, it
doesn't go beyond the login screen. I have tried changing
the password and deleting the password and still have the
same result.

I press start, log off, switch user and then make a
selection and it will not go beyond that point, almost as
if I have been locked out somehow. Any ideas? Two of the
accounts are listed as "computer administrator"
accounts. Please help. Thanks.