3.0 Upgrade Breaks Site: Failed opening required './admin-header.php'

I just used the automatic upgrade from my WordPress 2.9.2 site to the WordPress 3.0 stable build. First I disabled all plugins and renamed the plugins folder, and then created a new plugins folder (which is empty).

I am now getting a number of errors, but the fatal error that crashes my admin site is at the bottom:

Notice: register_sidebar_widget is deprecated since version 2.8! Use wp_register_sidebar_widget() instead. in /home/entechce/public_html/wptest/wp-includes/functions.php on line 3237

Notice: register_widget_control is deprecated since version 2.8! Use wp_register_widget_control() instead. in /home/entechce/public_html/wptest/wp-includes/functions.php on line 3237

Notice: register_sidebar_widget is deprecated since version 2.8! Use wp_register_sidebar_widget() instead. in /home/entechce/public_html/wptest/wp-includes/functions.php on line 3237

Notice: register_widget_control is deprecated since version 2.8! Use wp_register_widget_control() instead. in /home/entechce/public_html/wptest/wp-includes/functions.php on line 3237

Notice: register_sidebar_widget is deprecated since version 2.8! Use wp_register_sidebar_widget() instead. in /home/entechce/public_html/wptest/wp-includes/functions.php on line 3237

Notice: register_sidebar_widget is deprecated since version 2.8! Use wp_register_sidebar_widget() instead. in /home/entechce/public_html/wptest/wp-includes/functions.php on line 3237

Notice: register_widget_control is deprecated since version 2.8! Use wp_register_widget_control() instead. in /home/entechce/public_html/wptest/wp-includes/functions.php on line 3237

Notice: register_sidebar_widget is deprecated since version 2.8! Use wp_register_sidebar_widget() instead. in /home/entechce/public_html/wptest/wp-includes/functions.php on line 3237

Notice: register_widget_control is deprecated since version 2.8! Use wp_register_widget_control() instead. in /home/entechce/public_html/wptest/wp-includes/functions.php on line 3237

Notice: get_bloginfo was called with an argument that is deprecated since version 2.2! The home option is deprecated for the family of bloginfo() functions. Use the url option instead. in /home/entechce/public_html/wptest/wp-includes/functions.php on line 3321

Notice: has_cap was called with an argument that is deprecated since version 2.0! Usage of user levels by plugins and themes is deprecated. Use roles and capabilities instead. in /home/entechce/public_html/wptest/wp-includes/functions.php on line 3321

Notice: has_cap was called with an argument that is deprecated since version 2.0! Usage of user levels by plugins and themes is deprecated. Use roles and capabilities instead. in /home/entechce/public_html/wptest/wp-includes/functions.php on line 3321

Notice: has_cap was called with an argument that is deprecated since version 2.0! Usage of user levels by plugins and themes is deprecated. Use roles and capabilities instead. in /home/entechce/public_html/wptest/wp-includes/functions.php on line 3321

Notice: has_cap was called with an argument that is deprecated since version 2.0! Usage of user levels by plugins and themes is deprecated. Use roles and capabilities instead. in /home/entechce/public_html/wptest/wp-includes/functions.php on line 3321

Notice: has_cap was called with an argument that is deprecated since version 2.0! Usage of user levels by plugins and themes is deprecated. Use roles and capabilities instead. in /home/entechce/public_html/wptest/wp-includes/functions.php on line 3321

Notice: has_cap was called with an argument that is deprecated since version 2.0! Usage of user levels by plugins and themes is deprecated. Use roles and capabilities instead. in /home/entechce/public_html/wptest/wp-includes/functions.php on line 3321

Notice: has_cap was called with an argument that is deprecated since version 2.0! Usage of user levels by plugins and themes is deprecated. Use roles and capabilities instead. in /home/entechce/public_html/wptest/wp-includes/functions.php on line 3321

Notice: has_cap was called with an argument that is deprecated since version 2.0! Usage of user levels by plugins and themes is deprecated. Use roles and capabilities instead. in /home/entechce/public_html/wptest/wp-includes/functions.php on line 3321

Warning: call_user_func_array() [function.call-user-func-array]: First argument is expected to be a valid callback, 'remove_post_custom_fields' was given in /home/entechce/public_html/wptest/wp-includes/plugin.php on line 395

Notice: has_cap was called with an argument that is deprecated since version 2.0! Usage of user levels by plugins and themes is deprecated. Use roles and capabilities instead. in /home/entechce/public_html/wptest/wp-includes/functions.php on line 3321

Warning: require_once(./admin-header.php) [function.require-once]: failed to open stream: No such file or directory in /home/entechce/public_html/wptest/wp-admin/index.php on line 45

Fatal error: require_once() [function.require]: Failed opening required './admin-header.php' (include_path='.:/usr/lib/php:/usr/local/lib/php') in /home/entechce/public_html/wptest/wp-admin/index.php on line 45.

I think it has something to do with relative paths, but i’m not exactly sure how to resolve it. The wp-admin site is what is broken, because the main site works, just has a bunch of warnings to display.

Has anyone run into this problem? Or does anyone have any ideas? Thank you very much!