How Can I Use Task Scheduler to Have Reminders Pop Up on Screen?

Hello,

I would like to use Task Scheduler to help several family members improve their time management.  Three family members are on constantly on their PC either for work or pleasure.  Two are running Windows 7 while the other is on XP.  In all cases I am set up as the admin on their PC and they are a Power User.

I would like to schedule several tasks that will pop up messages on their screen at set times, every day.  For example, a message would pop up at 11 pm saying it is time to wrap up homework and get ready for bed.  Ideally, the task would do the following:

1. At a minimum, allow me to format the text of the message so that it is a relatively large screen font that cannot be missed.

2. Prefer if it will display an image (say a GIF or jpeg) that I would create.  The image would take up at least 50% of their screen.

3. The image/notice must be displayed on top of all open windows/browsers.  

4. Ideally, the user would not be able to close it.  The image would display for a set time - say 10 seconds, then close itself.  If this is not possible, it is OK if the user can close the image.

5.  The task should run when they are logged in and should not require that the admin be logged in.  It is OK if it runs for everyone.

I tried creating something on my own and it did not work.  It would be great if someone could explain step by step how to do this.

Thanks!
Chip

P.S.  I am open to the idea of using a third party application to do this, if there is a good, cost-effective solution.