Hmm, so the demo is now locked. Not so good. No way am I dropping $50 on a game if I can't tell if it'll run or not on my pc. I guess I'll have to wait a wee while before I can try it.
Well, being a Source game, it SHOULD run on most anything made in the past three or four years, and even longer before that if you spent a little on upgrades between here and now.
I'm running it on an AthlonXP 2500+, a Radeon 9600XT 128MB AGP, and 1GB PC-3200 RAM. It's not pretty, and it drops to about 16-22 fps in heavy areas, but it's still playable. Just make sure you're running the latest drivers for everything, and put everything on as low as it'll go. If you still want more frames, start dropping the resolution.
EDIT: Just a notice for people on the fence about picking up L4D - Valve's added L4D to their little "everything Source" superpack on Steam without increasing its price. It's still just $99 for every single Source game out there; there isn't a better deal in gaming anywhere, folks. If you already own one or more of the games, you'll be able to "gift" those extra copies to a friend, as long as they have a Steam account (they're free.) Buy the Source pack and give the gift of gaming to some folks that've been missing out