I use an old PC. My fiancé has a nice, more recent one with Vista (which I find to be okay but not great). I don't mind using Windows, when it works properly, but I personally prefer Macs. The reasons aren't that Macs are so much better, like some fanboys would say, but I like their OS and their systems. It has an intelligent and classy design. <3 They are expansive though, I won't deny it.
The games I play are available on Macs. So no problem there. I'll probably get a Macbook in some near future, unless Windows 7 and some awesome new (pretty) laptop comes into the market. That would *maybe* change my mind.
Edit: I wouldn't mind using Linux either, if it could run my games. -_-'