How to manipulate URL inside ajax call?

I am trying to trigger an ajax call on change of a dropdown menu(at client_script.php). The ajax call will send some values to server_script.php and it will change a div section of the client_script.php. The problem is the server script is kept in a local directory such as blocks/latestgrades/server_script.php. When the ajax call is done the url becomes the address of the server_script.php prepended by http://localhost/umoodle Which is quite ok, because the server_script/php file location is such. But when I hover around the other pages, the onchange triggeres the ajax call which tries to find the server_script.php on that location but prepended by http://localhost/umoodle/<something_i_dont_want_here>/ How to alter the url to cut that place I dont want and then do an ajax call with that?