One thing that I noticed, at least subjectively in my observations about XP (SP1), as compared to W2K SP2 - processes running at the same base priority level, and trying to chew all CPU, tend to cause "scheduler starvation" moreso than in W2K. I'm not sure why, but it sure is annoying. Two processes running at the same base priority, should get nearly equal CPU-time scheduled, I would think. (Even taking into account the priority boost given to the currently-active frontmost app, or an app with pending input queue messages.)
I've also come to believe that the reason behind some of the different behavior that I've seen seeing, is a result of the "tweaking" that MS supposedly did to the process working-set adjustment algorithm in XP. It seems as though the amount of physical RAM that gets allocated to a process, depends on whether it is an "active" task or not (minimized main window = "inactive" on XP), and some processes that were running fine while not minimized (even if not the front-most application), when minimized, XP trims their working-set so badly, that they start running primarily out of the pagefile, and performance of that app tanks. I'm really rather disappointed at the changes made to XP, by choosing to use whether or not the main window was minimized or not as a heuristic for allocation of physical RAM/working-set sizing, to the detriment of actual working-set usage and actual performance. Dave Cutler is an OS design Ghod among mortal software engineers; and thee lowly software-maintenance engineer shall not change what the hands of Dave have created. (IMHO, at least.)
W2K is definately better in these two regards.