Yes. They are dead or half-dead beings being controlled. (Headcrabs)
Quite an intriguing question, indeed! The word zombie itself denotes a reanimated corpse; in other words, a dead body that has, somehow, been brought to life. Now comes the question; are the human bodies that the headcrabs control still alive? As far as I know, this is open for speculation, as I have yet to hear a definite answer. However, it's likely that the headcrab - upon using its beak to pierce the cranium and incapacitate and control the central nervous system - kills the human host, and merely uses its body as an extension of the headcrab's own. So, yes, it's quite likely that the zombies really are zombies!
188.8.131.52 20:50, May 20, 2011 (UTC)
Some of them seem to be have muffled screams of pain as if the human victim still had some level of awareness. Certainly the headcrab itself wouldn't know enough to scream "Oh my god!" while on fire and the fact these sounds are muffled brings out signs that the victim is still partially conscious; certainly a fate much worse than being a reanimated corpse.