By design the XP kernel is actually faster than Leopard kernel in terms of raw speed, but that's pretty much where it stops. In terms of actual multi-tasking, networking, and memory management the Mach kernel of OS X (and iPhone) are incredibly more efficient. If you want to do a real life comparison to prove this, try opening up more than just one app at a time under XP, and then try to do the same under OS X. There will be a huge difference. Likewise, if you try to open one app at a time on a bare install of either OS, XP will be faster. The BSD sub-sytems of OS X are also pretty resilient to prolonged usage and have many self-optimization systems in place to maintain that speed. This however, is not the same with XP. So although you might start with a super fast XP install, the time it takes for it to slow down is much shorter than it would for OS X.
Ofcourse, no OS is completely immune to slowdowns/fragmentation, etc but even here OS X has a better management system than Windows. The most common fragmentation (not even a major issue unless you're a creative professional who tends to move massive data to and from your HD) problem with OS X is free space fragmentation. Other than that OS X does do some low level defragging of small files, but for the most part fragmentation on OS X is not an issue for the average user, although to say that OS X doesn't get fragmented is a lie. Windows (and especially XP) on the other hand are prone to both filesystem fragmentation as well as free space fragmentation, which is another reason why your Windows system will slow down much quicker than a similar OS X setup.
It's kind of like USB 2.0 (Windows) vs Firewire 400 (OS X). Sure USB has a theoretically higher transfer rate, but its transfer rate is highly inefficient in actually reaching anywhere near its limit, whereas even though Firewire's 400's top transfer rate is lower, it can actually reach its top speed while maintaing consistent read/writes.
