Dwarf Fortress if you can get past the lack of graphics. There's so much you can do, both in combat in Adventure mode (e.g. Aim an axe swing at a guy's left ear to piss him off), and building in Fortress mode (e.g. build a 60-story inverted pyramid full of cats, then pump water into it.)
Mount & Blade is an awesome series, revolving around travelling the lands doing whatever you want to do whilst many factions battle it out for supremacy over the world map. You can join any one of them and try to take the world for them, or you could form your own faction from scratch and take the world for yourself. Or you could be an ass like I always am, and raid villages, steal all their stuff, slaughter the populace and then sell their things to buy a new horse.
Also features a very interesting combat system, in which the way you move your mouse dictates what sort of strike you're going to pull off.
Features a levelling system with multiple stats to invest in, and is reasonably old so you should be fine running it on your PC. Start with Warband if you're interested.