It may be one of the best selling but so is Call of Duty. I'm fairly certain that amongst real gamers The Sims 3 isn't considered a good game.
@OP I'm assuming that seeing as you've come from consoles you'll be into FPS. You're going to need a Steam account (nearly everyone on the internet has one, in fact i'd be surprised if you didn't already). And I'd recommend pretty much ANY Valve game, Portal is the best IMO but Half-Life 2 if you want a FPS. Also, a few other games; Minecraft, Civilization (IV or V), TeS (any), RollerCoaster Tycoon 3 (some prefer old style of 1/2 though) and VVVVVV.
Aaaaand now I know enough about you to know that I shouldn't have bothered replying to you in the first place. I apologise for wasting your time.
I would still highly suggest that the original poster checks out The Sims franchise as it's excellent, and i'd even go against this fellow's suggestion that Call of Duty is rubbish. Because it's not. In fact, both Call of Duty 4 titles are bloody brilliant, if a little short. You can pick them up for fairly cheap and they're pretty decent on the PC too. If it's RPG you're after then Witcher 2 is still a good shout; if you're keen on a management-style RPG and you like football then there's no reason to miss the excellent Football Manager series of games. The latest can be picked up for cheap nowadays so it's definitely a good one to get.
I'd also put some thought to Telltale's recent point-and-click games. There's some real gems in there and you can get hours of enjoyment out of them for cheap...they're hardly graphics intensive too, so even if you don't have an amazing computer you can still give them a go!