want to show popup for registration

hi

I am doing programming in C# . I want to display popup using javascript/ajax when user click for registration or login. When popup is opened user can not able to do anything in background page (main page). Either user must be register/login or close that window then after able to do something in main page.

Also, once user login using this popup window then I want to refresh particular part of that page. Please let me know how to this possible ?

thanks