So, long story short, I uninstalled Steam from the C drive (93 GB ), cleaned up the registry, and reinstalled on the D drive (80 GB ). Half Life 2, EP1, EP2, and Portal all work fine.
Half Life…loads for a split second without the menu ever appearing (but you see the background image of Gordon with a shotgun) and then mysteriously crashes. Ditto for Blue Shift.
However…Opposing Force works perfectly and doesn’t crash.
Any idea what might be going on?