If you dont have Resident Evil,try this one.

User Rating: 7.5 | Cold Fear PC
Basically,It's a rip-off of the Alien movie saga.There are small exoskeletons that enter the human body through the mouth and horror begins.It also has a Resident Evil feel.
The game play is o.k. I guess,butt the aiming is bad and there is no map for the levels you move around in.There is definitely a spooky-scary atmosphere that sends the shivers down your spine.If you can look beyond the bad aiming system and buzzing around wondering where you should go next,it's a fair game.The characters are your basic: coast guard hero who saves the day,damsel in distress,crazy scientist that goes beserk and so on.
The graphics are spectacular (IMHO) and the environment responds very well to your actions (exploding barrels,valves,fire extinguishers...)