(Update: I was using an old, trial version of Nod32. Updated copies of Nod32 apparently did not make this mistake.)
At least that's what Nod32 thought.
I installed the demo via Steam and after it finished (2GB download!), I tried to launch the game.
Steam told me, "This game is currently unavailable. Try again later."
So I g-g-googled it, and the Steam knowledgebase told me to rename ClientRegistry.blob, to delete a bunch of files in the Steam folder, to verify the game cache...yada yada.
What the problem was, was that Nod32 quarantined the Left4Dead executable. I just happened to notice the quarantine take place when I tried reinstalling the game.
So, I set Nod32 to exclude the Left4Dead folder from scanning. Now it works.
Except for the stutter.
Also, Left4Dead is going to be a Really Fun Game.