Hello again there.
Actually, you piqued my interest in L4D modding with your previous question, so i've been looking over the Design Docs they've released, and it looks like you'll need to be able to load the L4D engine and game into the Source SDK in order to create levels that will be adequate for L4D.
Moral: Yeah, you'll need to have Left 4 Dead installed on the computer. Sorry.
I mean, you can definitely get the SDK without L4D, but it doesn't look like you can create maps for Left 4 Dead without the game installed.
For more information, including a link to the Level design Docs for L4D, check here:
http://forums.l4dmaps.net/f6/left-4-dead-tutorials-from-valve-t623.html
And more here:
http://forums.l4dmaps.net/f6/left4dead-mapping-resources-and-links-t70.html
Or anywhere else on http://forums.l4dmaps.net , really.
And really, fellas. The XBox and PC versions of L4D are very comparable. I prefer the PC version because of the more ready availability of nifty tools from Valve, and because I've given up on the console FPS. That doesn't mean the core experience of the PC version is better, just that I can't use an analog stick to aim worth anything. And I really, really can't. And it's not exactly like TF2, where XBox players got the shaft for the longest time. Hey, I wonder if that big TF2 update for the XBox ever got rolled out...
Anyway, just trying to say, don't be ridiculous. Same game, different platform.