//set specific default button when textbox has focus
//usage
//TextBox1.Attributes.Add("onkeypress", "return clickButton(event,'" + Button1.ClientID + "')");


function clickButton(e, buttonid){ 
      var evt = e ? e : window.event;
      var bt = document.getElementById(buttonid);
      if (bt){ 
          if (evt.keyCode == 13){ 
                bt.click(); 
                return false; 
          } 
      } 
}