You will need a javascript or Jquery code to do that. And the corresponding event should be on key up.
$(#txtSearch).keyUp(function()
{
var textVal=$(#txtSearch).val();
alert(' searching for '+textVal);
]);
I can provide you a full working code if you want it.. but this already solves I guess... and best way to check is, you can check here itself in CodeProject. As you keep on commenting, you can see that the preview is created on text changed event.. Simple like that.
Regards
Jashobanta