Well, doing what you want (I think) can be done using the Autologin module for drupal ( http://drupal.org/project/autologin ) however this is seriously not recommended for use, as this obviously means that usernames and passwords are visible in the URL, as well as in any access logs. Therefore, the project was discontinued, not even sure that it will work in the latest drupal version.
Unfortunately, since the drupal form-generating mechanism has XSS protection, it prevents sending directly to the login page and hopes for the best.
There are a number of existing Single Sign On (SSO) solutions for drupal, although most of them integrate with another existing CMS or similar.
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