if(window.addEventListener) {
  window.addEventListener('load', focusFirstInput, false);
} else if(window.attachEvent) {
  window.attachEvent('onload', focusFirstInput);
}

function focusFirstInput() {
  var bFound = false;

  // for each form
  for (f=0; f < document.forms.length; f++)
  {
    // for each element in each form
    for(i=0; i < document.forms[f].length; i++)
    {
      // if it's not a hidden element
      if (document.forms[f][i].type != "hidden")
      {
	// and it's not disabled
	if (document.forms[f][i].disabled != true)
	{
	    // set the focus to it
	    document.forms[f][i].focus();
	    var bFound = true;
	}
      }
      // if found in this element, stop looking
      if (bFound == true)
	break;
    }
    // if found in this form, stop looking
    if (bFound == true)
      break;
  }
}