no me funciono aqui dejo el codigo para que me ayudes gracias
<body background="clanfire_03.gif">
<div id="Layer1" style="position:absolute; left:66px; top:49px; width:427px; height:141px; z-index:1">
<form name="form1" method="post" action="proceso_form_detalles_factura_venta.php">
<table width="98%" height="117" border="0">
<tr>
<td width="51%"><strong><font color="#FFCC00" size="3" face="Arial, Helvetica, sans-serif">Numero
De Factura </font></strong></td>
<td width="49%"><input type="text" name="Num_Factura" value="<?echo $_POST[Num_Factura];?> "></td>
</tr>
<tr>
<td><strong><font color="#FFCC00" size="3" face="Arial, Helvetica, sans-serif">Codigo
De Producto</font></strong></td>
<td><input type="text" name="Cod_Producto"></td>
</tr>
<tr>
<td><strong><font color="#FFCC00" size="3" face="Arial, Helvetica, sans-serif">Cantidad
Vendida</font></strong></td>
<td><input type="text" name="Cantidad_Vendida" onkeyup = "sumar(this.form)" /></td>
</tr>
<tr>
<td><strong><font color="#FFCC00" size="3" face="Arial, Helvetica, sans-serif">Valor
Unitario</font></strong></td>
<td><input type="text" name="Valor_Unitario" onkeyup = "sumar(this.form)" /></td>
</tr>
<tr>
<td><strong><font color="#FFCC00" size="3" face="Arial, Helvetica, sans-serif">Valor
Total</font></strong></td>
<td><input type="text" name="Valor_Total"></td>
</tr>
</table>
<div id="Layer2" style="position:absolute; left:53px; top:145px; width:433px; height:29px; z-index:2">
<table width="94%" border="0">
<tr>
<td width="47%"><input name="Submit" type="submit" value="Registrar " onclick="form.target='_self' "/></td>
<td width="35%"><input type="submit" name="Submit2" value="Generar Factura" onclick="this.form.action='generar_factura.php'" /></td>
<td width="2%"> </td>
<td width="1%"> </td>
<td width="15%"> </td>
</tr>
</table>
</div>
<p> </p>
</form>
</div>
<div id="Layer3" style="position:absolute; left:88px; top:24px; width:365px; height:16px; z-index:3">
<div align="center"><font color="#FFCC00" size="3" face="Arial, Helvetica, sans-serif"><strong>Detalles
De Venta</strong></font></div>
</div>
</body>
</html>