in .net 2.0 , to create a pop up, you may use a literal control from code behind.

 1. put a literal control on page, set its Mode property to "encode" or "Transform"

 2. literal1.visible = false

within button onclick event

 3. literal1.text = "<script>alert('" & YourString & "');</script> "

 4. literal1.visible = true

the literal control executes the popup each time the button was clicked.