See the tutorial how to use the Task Scheduler in Vista. In the Task Scheduler, under the View Menu, make sure Show Hidden Tasks is checked. Then go down the list until you see Defrag. In the Window to the right of the tree view, right click on ScheduledDefrag and click Disable. Close the Task Scheduler. It shouldn't bother you anymore.

Everyone has their own favorite defragger but I use Auslogics Defrag mostly. Now and then I use the Windows defrag on the command line with the undocumented -b switch to defrag boot files, as in
C:>defrag c: -b

Good luck.