The first computer we had was an IBM PcJr. My friend had an apple IIc I've used Mac's, but I haven't messed with them since the early 90's. I wouldn't count the stints I did in the computer lab. Every mac we had in the computer lab seemed to be broken about 90% of the time for some reason or another. I haven't messed around with the new Mac OS but I hear it's pretty sweet.

I just work on a Win XP machine at home, built it myself. Not 100% top of the line, but it'll handle just about any game other than HL2. In fact, it could probably handle HL2, but you'd have the crank the game down quite a bit.