I've been installing all my games on my laptop, just realised I don't actually have very many.
It's a shame you don't like strat games - I bought Command & Conquer 3 (I've already got all the prequels) yesterday, it's an absolutely amazing game. (I also bought Halo 1 - I've been playing a pirate copy for that long I was starting to feel a bit guilty..
Civ4 is another good strat game... the best PC games are strat games... dunno why but I've always preferred my PS1/2/P for racing games and the like.
HL2 + CS: S is definitely a must. Quake and UT2004 are also kinda standard games that should be in your collection. As is Diablo 2 - someone already mentioned it.
Dungeon Keeper is kinda a mix between Strat and RPG. It's bloody legendary - but also about 10 years old.