Popup window comes again and again without clicking on any event

Hi, 
I want to call a window.open(//xyz.aspx) as a popup on <Asp:Button> Server click event .
 
 In .aspx file i am using the following code to call popup Window
<asp:TextBox ID="txtHidden" runat="server" BorderWidth="0" BackColor="transparent" onfocus Go to the complete details ...