Well, technically ESF doesn't require steam til 1.2.... so using your existing HL or retail CS cd and the key will work-- for now...
Steps for esf on steam:
1) Download and install the minimum STEAM installer from
www.steampowered.com
2) When STEAM is set up from the installation, go to your games list and right-click on "Half-Life" and then do "Install"
3) During the Half-Life installation, it'll ask you which mods you want to appear in your 3rd party list (note: the installation will auto-detect your mods that you have installed on the WON version of HL/retail CS)
4) Select your mods. Finish the Installation.
5) Download the ESF Steam Pack 2.0 from
www.esforces.com in their HELP section.
6) Copy the fmod.dll file from your OLD Half-Life folder located in your SIERRA directory and paste it in your NEW Half-Life folder inside the SteamApps directory which is inside the STEAM folder.
7) Enjoy ESF. You can enter through the games menu, or search for a server thru the server menu off the steam selection screen.