Personally, I feel the Overwatch is more likely to be a heavily modified human rather than a computer. Her voice seems too natural, especially when you compare it to things like GlaDOS. But who knows?
Humanoid Combine
The primary military force of the Combine on Earth is the humanoid Combine Overwatch (referred to by Doctor Breen as the “Transhuman arm of the Combine Overwatch”, although he could be referring to the entire Combine military presence on earth as the Overwatch and the ranks including modified humans are the transhuman arm.): humans who have been modified into “transhuman” (or “post-human”, as referred to by Dr. Kleiner) cyborgs. They are the most frequently encountered foes throughout Half-Life 2. This could also be another possible reason why they are called the Combine, because they are a “combination” of humans and alien technology. It is important to note that the Civil Protection foes are not hybrids and are in fact, human.
Civil Protection: The quasi-thoughtpolice that patrols the cities, monitor the public airways, and conduct randomized raids. Since little modification is involved, some Resistance members, such as Barney Calhoun, join Civil Protection in order to go under cover to investigate Combine activity and tip off would-be victims.
Overwatch Soldier: The main military of the Combine on Earth. Soldiers patrol and enforce Combine law outside of the city boundaries. It includes the Shotgun Soldier and the Nova Prospekt Prison Guard.
Overwatch Elite: The Elite soldiers of the Combine Overwatch. They’ve undergone extensive modifications and are seen protecting the Citadel or occasionally leading squads of regular soldiers.
Stalker: Mutilated, drone-like slaves that are created from dissident Citizens and captured Rebels.
Guys, that mean that I know ^^