Assuming you have defragmented the hard drive periodically, it will calm down...
This is often a problem with older computers using Windows XP and larger drives than the expected 40, 60, or 80 GB drives.
The Latitude D600 is not as bad as the D400 and D600... but once it becomes accustomed to the larger size, you should find it runs better... but watch out when it becomes more full and compacted... when it has only 50 or 40 gb left, it will slow down more.