You can't- it's tied to your Steam account now. Welcome to the brave new world.

Damn yeah you are right..

well, that put me off L4D and any future Valve/Steam products.

I hate the new world.

I don't particularly mind when I buy and download a game through Steam, but when a retail copy of a non-Valve game has the same restrictions and more importantly doesn't advertise that fact- not on.

That's what you get for buying crapware

I kid, I kid.

Seriously though Steam is great.  It's an extra step when you have to purchase from the store and then link to Steam, but it's like, 5 less steps if you purchase over Steam instead.  Just click to download and install, it'll take an hour or two, but then you're ready to go.  Very intuitive and hassle-free, although if you have internet issues, I can see it being a pain.