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HL2 enjoys having Steam set up even if you're playing offline, but if you really hate Steam for some reason (perhaps it killed your father), you can run it perfectly well sans Steam. I dunno how easy it will be to convince HL2 you don't want Steam at all, but if you disabled Internet when you installed, that would probably do it.
I'd still suggest that you get a Steam account, and even buy it through Steam. Right now it's discounted, and either way, if you've got it tied to Steam, you can install it anywhere you want without the discs, and you don't need to worry about losing the discs or anything. Plus you can make as many backups as you need.
I think it's pretty close to required: I believe HL2 tries to install Steam when you install it. If you don't let it get to the Internet, though, it can't do anything, which suggests you can go Steam-free even if you do have Internet. You might just need to be crafty.
Then again you won't get any patches for HL2, and you won't get Lost Coast for free, and you won't get all the other benefits I mentioned. So Steam's definitely the way to go.