try it at your own risk 
source : http://www.theinquirer.net/?article=24749

source : http://www.theinquirer.net/?article=24749
The only reason why its faster is they added a superfetch feature to the prefetcher. If you look at the key: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Memory Management\Prefetcher you will notice in windows xp EnablePrefetcher = 3 and you will notice in windows longhorn EnableSuperfetch = 1 Well, guess what? You can put the EnableSuperfetch = 1 in windows xp and get the same speed. Wow, Microsoft just added a feature that was already there in xp. snakeye