Rendering drupal login form in a modal window
Modal forms module might help you.

Modal forms make use of the modal feature in the ctools module to open some common forms in a modal window.
Supported forms:

Log in (modal_forms/nojs/login) Request new password
(modal_forms/nojs/password) Create new account
(modal_forms/nojs/register) Contact (modal_forms/nojs/contact)
Comment (modal_forms/%ctools_js/comment/reply/%node) Webform
(modal_forms/%ctools_js/webform/%node)