At times you on the server side, after a web page has posted back, you might want to open a popup window.  Since the control is now on the server side, you must send some javascript code back to the client to opent he window.  Here is a simple example on how to use Response.Write to perform this task:

Response.Write("<script language='Javascript'>"+
     "window.open('MyPageToOpen.aspx?')"+
     "</script>");

You can also provide more parameters on the "window.open" function to specify window size, menus, etc.