... con questa piccola modifica:

codice:
function validateCheckbox(formObj, minC, maxC) { 
var objName = formatName(formObj); 
var formObj = formObj.form.elements[formObj.name]; 
var checkTotal = formObj.length; 
var checkCount = 0; 

if (maxC == 999) maxC = checkTotal; 
for (var i=0; i<checkTotal; i++) { 
if (formObj[i].checked) checkCount++; 
} 
if (checkCount < minC || checkCount > maxC) { 
if (fv['groupError']) { addError(formObj); return true; } 
alert('Please select between '+minC+' and '+maxC+' options for '+objName+'.\nYou currently have '+checkCount+' selected'); 
for (i=formObj.length-1; i>=0; i--) 
errorProcess(formObj[i],0,1); 
return false; 
} 
if (checkCount==0) {
alert("Almeno un checkbox deve essere selezionato.");
return false;
}
return true; 
}
Fammi sapere !