Topic on Engine talk:Source

For some reason most of my Source games crash after the "Preparing to Launch" screen. Half-Life 2 (and the HL episodes), L4D, L4D2, Portal, and Portal 2 all immediately "encounter an error and need to close". The only Source games that do work are Counter-Strike: Source, Garry's Mod, and Team Fortress 2 are the only ones that do work. I've contacted Steam Support to no avail and I'm mostly just finding dead threads with same problem. I've tried doing what the Steam forum says and using CPU-Z to find out if my computer has SSE support but that program has not worked. I'm becoming extremely frustrated and would appreciate any help at all.