Ok, what you need to do is listen to people's answers. I'm going to give you a quick run down of what you need to do.
Since you're using steam, here's what to do.
Delete your ESF folder entirely. It should be found in C:\program files\valve\steam\SteamApps\youraccount@email\half-life\
So delete the folder called ESF. Then run half-life. Just execute from the steam menu, and then exit once it loads. Then reinstall ESF 1.2 (Main installer) and 1.2.1 (Patch), 1.2.2 (Patch), and 1.2.3 (patch). So that's 1 main installer (the big file) and 3 smaller patches. Once you've done this, everything SHOULD work fine. If not, then put the fmod.dll file in the ESF directory from the link above and Please, read the FAQ that was posted.
Do all of this BEFORE you ask ANY other question, understand? No more questions until you've done what I've told you to do.
*edit* here's 2 links for a full installer. Shouldn't need the patches with this.