<input type="text" onkeydown="numberOnly(event)" />
function numberOnly( event )
{
  var key = event.keyCode;
  if ( !( ( key > 95 && key < 106 ) || key == 8 || key == 9 || key == 37 || key == 39 ) )
  {
    if ( window.event ) //IE
    {
      event.returnValue = null;
    }
    else //Firefox
    {
      event.preventDefault();
    };
  };
};