ciao io ho questa funzione(ti do anche quella x eliminare la riga):
codice:
<script type="text/javascript">
	var cntRows=1;
	function AddRow() {
		cntRows++
		var cell=cntRows
		var newcell="<input type=\"text\" name=\"riga"+"\" size=\"7\" value=\""+cell+"\">"
		var newcell2="<input type=\"text\" name=\"art"+"\" size=\"20\">"
		var newcell3="<input type=\"text\" name=\"qnt"+"\" size=\"20\">"
		var newcell4="<input type=\"text\" name=\"prezzo"+"\" size=\"20\">"
		var newcell5="<input type=\"button\" name=\"B2"+"\" size=\"20\" value=\"Inserisci Riga\" onclick=\"AddRow()\">"
		var newcell6="<input type=\"button\" name=\"B3"+"\" size=\"20\" value=\"Elimina Riga\" onclick=\"DelRow()\">"			
		
		tDettaglio.insertRow()
		tDettaglio.rows(tDettaglio.rows.length-1).insertCell()
		tDettaglio.cells(tDettaglio.cells.length-1).innerHTML=newcell
		tDettaglio.rows(tDettaglio.rows.length-1).insertCell()
		tDettaglio.cells(tDettaglio.cells.length-1).innerHTML=newcell2
		tDettaglio.rows(tDettaglio.rows.length-1).insertCell()
		tDettaglio.cells(tDettaglio.cells.length-1).innerHTML=newcell3
		tDettaglio.rows(tDettaglio.rows.length-1).insertCell()
		tDettaglio.cells(tDettaglio.cells.length-1).innerHTML=newcell4
		tDettaglio.rows(tDettaglio.rows.length-1).insertCell()
		tDettaglio.cells(tDettaglio.cells.length-1).innerHTML=newcell5
		tDettaglio.rows(tDettaglio.rows.length-1).insertCell()
		tDettaglio.cells(tDettaglio.cells.length-1).innerHTML=newcell6
	}
		
	function DelRow() {
		var current = window.event.srcElement;
   		while ( (current = current.parentElement)  && current.tagName !="TR");
        		 current.parentElement.removeChild(current);
	}
dove la prima riga e i pulsanti sono:
codice:
	<td><input type="text" name="riga" size="7" value="1" readonly></td>
	<td><input type="text" name="art" size="20"></td>
	<td><input type="text" name="qnt" size="20"></td>
	<td><input type="text" name="prezzo" size="20"></td>
	<td><input type="button" name="B2" onclick="AddRow()" value="Inserisci Riga"></td>
	<td><input type="button" name="B3" onclick="DelRow()" value="Elimina Riga"></td>