That part isn't too surprising. I'm just wondering if it's because Windows 7 is a better OS or if it's because we have much more capable hardware these days (and Windows 7 is designed to take advantage of that hardware, while Windows XP isn't).

To give you some examples: SMP was pretty rare when XP was released and Hyper-Threading wasn't introduced yet, so there was very little point in optimizing XP for concurrent processes. XP also put most of the graphics rendering onto the CPU, possibly because a lot of 2001 vintage low end video cards weren't much better than a frame-buffer at the time. That's just the tip of the iceberg, since there have been an absurd number of changes to PC hardware architecture over the years, none of which an old OS will take advantage of simply because it doesn't know how to use it.