this one works

function fillUpSelects2(start, top, who) {
    var optionArray = new Array();
    for (var i = start; i <= top; i++) {
        optionArray.push(new Option(i, i));
    }

    var selects = jQuery("select.estanciaselect");

    selects.each(function(){
        if(jQuery(this).attr('id') != who){
            jQuery(this).html('');
            for (var j = 0; j < optionArray.length; j++) {
                jQuery(this).append(jQuery('<option>').attr('value', optionArray[j].value).html(optionArray[j].text));
            }
        }

}

I'm sorry I posted this so late, I was having some trouble with my internet.

Anyway I got to fix the problem. I don't really know why it got fixed, I just created a StreamWriter and passed ti to the StreamResult constructor and it worked.

Here's the fixed code.

Thanks for your help.

if (f.exists()) {
	    StreamWriter sw = new StreamWriter();
            StreamResult result = new StreamResult(sw);
            DOMSource source = new DOMSource(doc);
            trans.transform(source, result);// here's where the Exception gets thrown
        } else {
            throw new Exception("The file doesn't exist");
        }

Thanks man you saved my day. I guess I gotta learn to search the net, laughing.

do I have to tell the request that I'm writing an image into it?

that would be a resource, but I´m telling you I´m finding really hard to find some information that could help me fix this problem. Thanks for the suggestion.