Intendo dire disabilitare l'href del link od anche sostituire alla icon.gif un'altra icona senza alcun href.

Ad ogni modo ho provato il tuo codice e qualunque valore seleziono dalla select name="t_im" si apre la pagina figlia in popup ed il link su icon.gif della pagina madre è sempre attivo:

codice:
<html>

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

<script type="text/javascript">

function CheckIcon1(ss){
	if(ss.options[ss.selectedIndex].value==''){
		document.getElementById('icon1').onclick=Function('return false')
	}
	else{
		document.getElementById('icon1').onclick=Function('return true')
	}
}


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="Form();CheckIcon1(this)">
<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()" onclick="return false">
[img]icon.gif[/img]</a>
            
<select size="1" name="c_fu">
<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="icon1" href="javascript:Form()" onclick="return false">
[img]icon.gif[/img]</a>
            
</form>

</body>

</html>