The only reason that the next Windows version (codenamed Longhorn) will be faster is because they added a superfetch feature to the prefetcher. If you look at this system key:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Memory Management\Prefetcher
you will notice in windows xp that it says:
EnablePrefetcher = 3
and you will notice in windows longhorn it says:
EnableSuperfetch = 1
Just put the EnableSuperfetch = 1 in windows xp and get the speed that longhorn will have.
If you are not familiar with system keys and altering them, I suggest that you do not attempt the above. Anyone who actually uses this information does so at their own risk and I am not liable for anything that goes wrong with your computer.