The disk defragmenter in Windows Vista was redesigned.  I personally do not like the new stripped down interface but one welcome addition is easy defrag scheduling. 

Click on the Start Button and type in Disk Defragmenter and hit Enter

Make sure Run on a schedule is checked as shown above.

Click Modify schedule and configure the settings for your computer use.  I like to set mine to Daily and late at night since I normally leave my PC on at night.