Ars Technica has a good article on why Windows XP may be slower when using brand new hard drives: http://arstechnica.com/microsoft/news/2010/03/why-new-hard-disks-might-not-be-much-fun-for-xp-users.ars
It boils down to bigger hard drives need more error correction space to keep your data intact; and so, as hard drives get bigger, it becomes harder to maintain the integrity of your data.
The solution is to change the size of the storage areas on the hard drive so that it is easier to maintain the integrity of the data.
Because Windows XP was created before this was an issue XP has to perform more actions to read and write data of off these newer hard drives, which makes everything slow down.
This is another good reason to move on to Windows 7 as soon as possible.