Can a form element be disabled by form_alter hook

I am in process of working ona registration form modification i want to DISABLE some fields of a form by using the form alter hook
I NEED 2 DIFFERENT form views
1 having 5 fields
2 having 10 fields

How i want to do is i have made extra fields using the Profile Module
Now when i call the registration form by

http://localhost/drupal/user/register

It returns all the fields

Wht i want to do is By default Disable some fields so that the validation can also be skipped for MANDATORY fields and on click of a button those fields are given to user with working validation

Can Some 1 help me with this
1.)How to disable fields using user_form_alter() i have links showing how to add LABEL or New field but no option of How to disable fields

Below is link for form alter function i have followed
http://drupal.org/node/19855