check out the 2nd page of posts for others with the same problem... i posted this suggestion, and nobody has responded, maybe it works?
try uninstalling (not disabling) alcohol 120 or whatever virtual cd-rom/dvd-rom app you are using to mount images; I have had the same problem with several recent games (doom3 etc) and found that to be the source of the problem. Dont have HL2 yet so who knows, but it's worth a try.
