Open user login in lightbox
Or try using Ajax Login/Register, a complete solution. Here is a quote about it (from the module's project page):

... provides a smooth way for the user to login, register and request new password via AJAX at any Drupal site.
Features:

This module provides a block with 2 links (Login, Register) for the anonymous user.
When the user clicks on Login or register the requested form appears in a nice ajax popup.
The login and register forms can validate using ajax and not redirect on error.
On success the module redirects to the user page.
When user is logged in, the block provides a Welcome !username message and a link to the user page and a logout link.
This module integrates the thickbox plugin with the Ajax module and provides a simple, easy to use and cool login and register block.