KNiVES' guide to classical and good old PC gaming:
-Half Life series (Orange Box) as a staple bread and butter of PC FPS.
-Battlefield series as your typical squad/large scale multiplayer FPS.
-Counterstrike for competitive multiplayer FPS.
-CoD4 offers alternative single and multiplayer experiences.
-Give Company of Heroes, World in Conflict and Supreme Commander a whirl as those are popular and staple RTS PC games.
-GTR for your typical PC racing simulator, or GRiD if you want something a bit more arcadey.
-If you have vast vacuums of time to spare, try World of Warcraft if you've never played an MMO before and are curious.
-Stalker should be looked at as a special case in PC gaming not only for its near-mythical status due to protracted development but because it's actually a good game.
-Oblivion and Morrowind should be considered if you like freeform RPG games. Both are PC classics.
-Unreal Tournament (any) and Quake II/III for your quintessential fragging pleasure.