Need your help in flashing asp.net web application window

We have application session time out set as 20 minutes
We need to display message box after 15 minutes saying your session is about to expire and if user is in any other window then the application window should flash. We tried code from few forums but flashing application window is not working.

We need to implement this functionality in asp.net web application (C# language, .net framework 3.5)

Kindly reply to me if you have any solution.