Actually, the error popped specifying that the scheduled tasks is trying to use a username that doesn't have sufficient rights. Same thing happens when you want to delete some sche'd task.
Well, Vista is a junk scrap.
Anyway here's a screencap:
Possible Solution:If RUN Whether User is Logged on or not option is used
>>>Check the box on Do Not Store Password
>>>Check Run at Highest Privileges
>>>Make sure you got sufficient Admin rights for what your tasking.