System Shock may be a little old if you're not used to old DOS games. If you're quite new to PC gaming, you could try Fable which is really a hybrid console/PC RPG or the Witcher if your PC handles it.
Baldur's Gates 1&2 have maybe the best storytelling among PC RPG.
Otherwise I would suggest not to limit yourself to RPG/Adventure Games since a lot of unique games came out on PC.
Trackmania Nations: free online racing game
Portal: First Person Puzzle
Heroes of Might and Magic: Turn based fantasy strategy game; very relaxing and addictive. Try HOMM3, its a classic.
SWAT4: Tactical Squad FPS where you almost never shoot
Racing games: The best simulations are on PC (Richard Burns Rally, F1 Challenge 99-02 (old but good), GTR2, rFactor (a bit hardcore))







