You can't stop the task scheduler. From Technet ...
The Task Scheduler service cannot be
stopped or disabled by administrators
using the Services Microsoft
Management Console (MMC) snap-in user
interface, unless special permissions
are added to the administrator token.
There are some registry tweaks that claim to get it to stop, but your best bet is to use a tool. Security Explorer has a trial version that should work for you. If you want to prevent users from scheduling tasks, there is a way.
The best registry entry I found was below. But I don't have a Vista machine to test it on.