Also if you're able to get some money go get House of the Dead 2 (it was one of my favourate games as a child), the first 3 resident evil's were also pretty good, Director's Cut (doubt it's on pc) if you have to :/
Some people I was talking to mentioned Amnesia though I haven't played that yet so can't tell you anything about it, I know there's a free demo out so check it out man.
Silent Hill, Alone in the Dark 1 and 2 (not really horror but almost)
Amnesia the dark decent.
Make sure you get : no light anywhere except monitor + headphones or good 5.1 sound system.
With my hi-end headphones that game scared tha shit out fo me.
I already named it, although as far as I know and I did mention this it's not free... unless you do it illegally which is not allowed on this site so yeah...