Allora ho trovato questa roba da forum di hardocp (
http://www.hardforum.com/showthread.php?t=833866):
"ok here ya go... create a shortcut of hl2 on ur desktop... right-click on hl2 in the "playgames" list and select create desktop icon.. under the desktop icon... click on properties and go to target and add -dx 90 at the end and it will force directx 9.0b.
But for the outcome.. u WILL loose about 50% performance and it will probably kill ur gaming experience unless u lower the settings or go down a resolution... i do not recommend doing this though.."
Lo stesso forum e altri che ho trovato rimandano tutti ad una review di anandtech del 17 novembre 2004 che tratta appunto del forzare le dx9 in hl2, ma non riesco a trovarla...