Have you tried updating the drivers for both sound and video? Where you are seeing it crash to the desktop in a game like HL2 you could be seeing a driver problem especially sound or a disconnect from the game servers since you have to log into an online account to run it. A shaky supply would see more then a crash to the desktop. When going to run the game bring up the task manager ahead of time and end other unnecessary processes seen under your user name there to see if the same problem persists.