You re-place it within a function that uses the variable/vector screen_size.y and screen_size.x, there is an example in the manual:

http://www.conitec.net/beta/ascreen_size.htm

http://www.puppenheim.org/
http://www.wuerfels-welt.de
Old Game Tutorial: http://www.puppenheim.org/MITutorial.zip
Free Game Editor: http://puppenheim.org/AlienalsWeihnachtsmann.zip
http://www.puppenheim.org/blender_starting_main_settings.txt