Fiddling with the compatibility settings was actually one of the first things I tried. The result was that STEAM complained about not needing compatibility adjustments, and CSS still didn't work. I'm 60% sure it's a problem with the CSS files and a delete/reinstall will fix it. I'm just sick as a dog and not really feeling up to the hassle of doing the work quite yet. I'll probably setup the reinstall, tonight, though.
[Edit]
No dice. I deleted all local content and reinstalled CSS, but to no avail. I also updated the software on my 3COM 3CR858-91 modem and tried to make sure all required ports were open. I'm not sure I succeeded, however. I'm not an expert on port forwarding, and the 3CR858-91 has a weird sort of setup.
Oh, and to make things even more interesting, I tried running hl2.exe. I got a popup message telling me some file or other was too old, and then the game updated it and ran flawlessly. Go figure....
[Second Edit]
After screwing around a bit more, I discovered that I can PLAY CSS, I just can't host a map. That includes the "video test" map (which is how all this got started). If I join a server, no problem. If I try to host a map or run the video test, crash-a-mundo.
-Khalaq