plugins_url function mixes system path and URL

Question

In my WordPress widget I use the following code:

wp_register_script('jquery-ui.widget', plugins_url('assets/js/jquery-ui-1.9.2.widget.js', dirname( __FILE__ )));

Unfortunately, the code gives me an invalid URL which is mixed with my system path:

http://test.dev/wp-content/plugins/C:/projects/wordpress/plugins/assets/js/jquery-ui-1.9.2.widget.js?ver=3.6

Is this problem maybe related to the fact that I have a local WordPress installation running on a Windows computer (with Apache 2)?