Look in the /templates/user/ subfolder of the ezwebin extension (presuming you have that installed). It has 7 templates in it that control most aspects of the registration and login process. You can override these easily by creating a template with the same name/location in your design folder or extension.
Depending on your needs, you may have to go a little deeper into the datatype templates or attribute edit templates to control the behavior of parts of the form.