I have a similar problem, but this is with XP. I installed Opposing Force onto Steam and its been running perfectly, but then at one stage on... load of4a3? it instantly quits out of the game, weird thing is, only when i reach a certain height in the room (as your suppose to use the barnacle to grab onto something to get to the vent half way up the wall so you can get to the next load), otherwise i can move all around it. Reinstalled off CD, redownloaded off steam, have tried EVERYTHING i can think of and yet it continues to be a pain in the ass, HL1 and the "Platinum Pack" seem to work off steam but the resolution settings don't suit my screen so i don't use it, this is really weird, i'm thinking probably a corrupted file or incomplete install, but then why does it come up with 100% - Ready in steam and not need an update? Any tips are greatly appreciated - i'll keep workin on it anyway