If you are unable to select any users on the login screen, you may need to
boot into safe mode so that you can log in as the administrator. Try
hitting F8 right after your machine Posts (beeps) when you reboot. Then
select safe mode. Chances are that your administrator password is blank, so
just enter administrator as the username, and leave the password field blank
(or enter the real password if you set one).
You can then follow the instructions that Michael sent regarding setting up
a new account and migrating the settings from your old account.