It has been my opinion for quite some time that defraging your hard drive every week it overkill and too intensive on your hard drive. I have no idea if the way Vista defrags your hard drive is with less stress on the drive than XP but Vista by default automatically defrags your drive every week. At this point, until I have more information, I recommend you modify the setting to defrag hard drive to only once a month. To do this, right click on your hard drive, select "properties," then the "tools" tab, "defragment now," and finally "Modify Schedule." In the Modify Schedule box which is called Disk Defragmenter, you can select a different schedule and it should change the schedule for all drives you have in your computer.
If you don’t want your computer to defrag your drives on a schedule, just uncheck the box, "Run on a Schedule (recommended)," when in the Disk Defragmenter dialog box is open. When you want to do a defrag, just go back to the Disk Defragmenter dialog box and select, "Defragment Now."