beh... devi mettere id diversi e fare un'altra funzione
codice:
<html>

<head>
<meta http-equiv="Content-Type" content="text/html; charset=windows-1252">

<script type="text/javascript">

function CheckIcon1(ss){
	var ok=false
	if(ss.options[ss.selectedIndex].value==''){
		document.getElementById('icon1').onclick=Function('return false')
		document.getElementById('img_icon1').src='/mac/images/elimina_icon.gif'
	}
	else{
		document.getElementById('icon1').onclick=Function('return true')
		document.getElementById('img_icon1').src='/mac/images/modifica_icon.gif'
		ok=true
	}
	return ok
}

function CheckIcon2(ss){
	var ok=false
	if(ss.options[ss.selectedIndex].value=='NO'){
		document.getElementById('icon2').onclick=Function('return true')
		document.getElementById('img_icon2').src='/mac/images/modifica_icon.gif'
		ok=true
	}
	else{
		document.getElementById('icon2').onclick=Function('return false')
		document.getElementById('img_icon2').src='/mac/images/elimina_icon.gif'
	}
	return ok
}


function Form(theUrl, percSize) 

{

var width = 400; 
var height = 200; 
var left = Math.floor((screen.width-width)/2); 
var top = Math.floor((screen.height-height)/2); 

  window.open(theUrl,'popup','width='+width+',height='+height+',top='+top+',left='+left);
  
}

</script>

</head>

<body>

<form action="form.asp" method="post" onsubmit="return(validateForm(this));">
 
<select size="1" name="t_im" onchange="if(CheckIcon1(this))Form('test.htm', '');">
<option value="">Select value</option>
<option value="A">A</option>
<option value="C">C</option>
<option value="Ca">Ca</option>
<option value="Cav">Cav</option>
</select>
            
<input type="text" name="e_gu" size="10" readonly>           
<a id="icon1" href="javascript:Form('test.htm')" onclick="return false">
[img]elimina_icon.gif[/img]</a>
            
<select size="1" name="c_fu" onchange="CheckIcon2(this);">
<option value="">Select value</option>
<option value="SI">SI</option>
<option value="NO">NO</option>
</select>
                  
<input type="text" name="c_ma" size="10" readonly>           
<a id="icon2" href="javascript:Form('test.htm')" onclick="return false">
[img]elimina_icon.gif[/img]</a>
            
</form>

</body>

</html>