Set Account Information dialog (Task Scheduler 1.0 on Vista SP1)

I created a task using task scheduler 1.0 interface, following the example in
MSDN, with the following instruction to set the "run only if logger on"
property. The user is the current user and password is null.
spTask->SetFlags(TASK_FLAG_RUN_ONLY_IF_LOGGED_ON);
spTask->SetAccountInformation(xxx, NULL);

After that, I called
spTask->EditWorkItem(hWnd, 0);
to show up a property sheet to let the user change time.

It works properly on XP and Vista, but with Vista SP1, after the user
changed the time and clicked OK, there was a strange "Set Account
Information" dialog box showed up. It was weird because I had already set the
"TASK_FLAG_RUN_ONLY_IF_LOGGED_ON" flag, so the task didn't need to store a
password.

And further more, that dialog doesn't actually do anything, it accept any
user name and password without checking them.

Any suggestion/information is welcome.
Thanks.

Correct the above post, the dialog box actually CAN change the user of a
task. But it doesn't notice you if you input the wrong password.

I found on XP/Vista, the dialog only show up if the user name and password
are not set. Once they are set, the dialog won't appears again. But on Vista
SP1, it appears everytime if you modified the task.