uninstall STEAM. Download STEAM. Install STEAM. Uninstall ESF. Install ESF into the STEAM folder. Not Sierra.
For the record, when you install Half-Life. It automatically wants to install into C:\Program Files\Sierra. You have to tell it to install into C:\ Program Files\valve\Steam\I am not at my\computer so I don't remember\the exact location\i'm cool
It's long, and annoying to remember. But that's where it goes. It's something like valve>steam>steamapps>Halflife or something like that. You'll find it. Make sure Half-life is in there, once you do, try to play half life once. Once you do that, if esf is in there it should be playable. But it might ask to download 420mb worth of nothing. Which even on a dial-up computer takes like 2 seconds.