Вот так должно работать, если конечно элемент с id="forData" у вас на странице один.
<script type="text/javascript">
window.onload=function(){
var div = document.getElementById('forData').getElementsByClassName('file laptop')[0];
var spans = div.getElementsByTagName('span');
for(i=0; i<spans.length; i++) alert(spans[i].innerHTML)
}
</script>
<div id="forData" style="display:none">
<div id="Toshiba_Satellite" about="#laptop" class="file laptop" typeof="gr:Offering">
<span property="dc:title">Toshiba Satellite</span>
<span property="rdfs:screen">15"</span>
<span property="rdfs:cpu">Intel Celeron Dual Core T3300</span>
<span property="rdfs:mem">2Gb</span>
<span property="rdfs:dimensions">383*259*27мм</span>
<span property="rdfs:battery">Литий-ионная</span>
<span property="rdfs:wrnt">1 год</span>
<span property="commerce:amount" datatype="xsd:decimal">3900 грн</span>
</div>
</div>