The Zombie's in H-L are Scientists with a Head Crab on it (In Opposing Force, there are also Barney and Human Grunt Zombies). They haven't changed much.
The oldest form of the Zombies that I know of, the only real differences is that more detail was added
This is pretty close to the current version, but for some reason they are blue
From this pic you can tell much more detail was added
This pic shows some more details were being added before it was finished
Another pic of a low detail Zombie