yeah, then in the future if you lose the cd or something, steam will know that you had a legit hl, so then you can dl it for free later, but that's just later.
Just install your hl under a directory similiar to this:
there where the bold "username" is typed is the folder with the rest of your games, so install it under there, then do what grega said and insert the cd key where needed, then install esf under the hl directory and I think that's it!
Do i have to have a certian version of half-life to run it??? Is retail fine? And i have 2 "versions" of hl 1 is modded and the other 1 is installed of the origanil disk! They have different menus!!!!!!! :?