I think someone on AVP Galaxy was working on a Prometheus fangame. I'll look it up for you.
Meanwhile, if you're POOR and like being ILLEGAL like ME, you can just get EPSX2 and an ISO of Extinction. I don't think Steam has it for PC.
I don't know big the modding community is for the AVP games on PC, but you might find some good mods that completely change the game (or at least add "new" missions or character skins and stuff). You can probably even find AVP mods for other games, like Left4Dead or Half-Life 2.
Edit: Look through this page
, it links to some fangames on AVP Galaxy.