1. Make pop up:(as the Muralikrishna88115 has created using the div)
2. Make a timer (timer1) in aspx page by dragging the Timer control from toolbox
3.write the following where u want to open the popup (eg. In PageLoad):
System.Timers.Timer time = new System.Timers.Timer();
time.Start();
then
4.Add the tick event to timer
5.in the tick event write the code to open the popup.
eg:divId.attributes.add("display","block");