Ti abbino il codice così puoi verificare, perchè anche con la tua modifica non sortisce risultato:
<%
Dim Conn
Set Conn=Server.CreateObject("ADODB.Connection")
Conn.Open "driver={Microsoft Access Driver (*.mdb)};dbq="& server.MapPath("corsi_formazione_Nordiconad.mdb")
'pdv=request.querystring("pdv")


cod_Pdv=replace(Request.Querystring("cod_Pdv"),"'" ,"''")


Session("cod_Pdv") = pdv
%>

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>Elenco Corsi</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
</head>

<body topmargin="0" leftmargin="0">
<table width="60%" border="0" cellspacing="0" cellpadding="0">
<tr>

<td height="562" valign="top"> <p align="center"><font face="Verdana, Arial, Helvetica, sans-serif">CORSI
DI FORMAZIONE 2006

Elenco corsi per mese</font>



</p>
<form name="form1" method="post" action="">


<font face="Verdana, Arial, Helvetica, sans-serif">Scegli il mese</font>
<select name="mese">
<option>01</option>
<option>02</option>
<option>03</option>
<option>04</option>
</select>
</p>
</form>
<p align="left">



<% SQL="SELECT * FROM Query_corsi_per_mese order by data asc"

Set rec = Server.CreateObject("ADODB.Recordset")
Set Rec=Conn.Execute(SQL)
While Not rec.EOF%>
<%
if form1= "01" then
SQL="SELECT * FROM Query_corsi_per_mese WHERE mese='01'"
end if
%>
<p align="left">

<font size="2" face="Verdana, Arial, Helvetica, sans-serif">Data
corso: <%=rec("data")%></font> </p>

<table width="100%" border="1" cellpadding="0" cellspacing="0" bordercolor="#FF00FF">
<tr>
<td colspan="3" bgcolor="#FF00FF"><font color="#FFFFFF" size="2" face="Verdana, Arial, Helvetica, sans-serif">Corso:
<%=rec("tipo_corso")%> </font></td>
</tr>
<tr>
<td width="46%" colspan="3"><font size="2" face="Verdana, Arial, Helvetica, sans-serif"><font color="#0066FF">Dettagli:</font></font></td>
</tr>
<tr>
<td><font size="2" face="Verdana, Arial, Helvetica, sans-serif"><font color="#000099" size="1">Tipologia
del Corso:</font><font size="2" face="Verdana, Arial, Helvetica, sans-serif">
<% = rec("tipo_corso") %>
</font></font></td>
<td><font size="2" face="Verdana, Arial, Helvetica, sans-serif"><font color="#000099" size="1">Titolo:
<% = rec("titolo") %>
</font></font></td>
<td><font size="2" face="Verdana, Arial, Helvetica, sans-serif"><font color="#000099" size="1">Argomento:
<% = rec("sottotitolo") %>
</font></font></td>
</tr>
<tr>
<td><font size="2" face="Verdana, Arial, Helvetica, sans-serif"><font color="#000099" size="1">Sede:</font>

<% = rec("Sede_Corso") %>
</font></td>
<td><font size="2" face="Verdana, Arial, Helvetica, sans-serif"><font color="#000099" size="1">Area:</font>
<% =rec("Area") %>
</font></td>
<td><font size="2" face="Verdana, Arial, Helvetica, sans-serif"><font color="#000099" size="1">Durata:</font>
<% =rec("durata") %>
</font></td>
</tr>
<tr>
<td width="12%"><font size="2" face="Verdana, Arial, Helvetica, sans-serif"><font color="#000099" size="1">Costo
Corso:<% = rec("costo_corso") %>

</font></font></td>
<td width="11%"><font size="2" face="Verdana, Arial, Helvetica, sans-serif"><font color="#000099" size="1">N&deg;
massimo di partecipanti:<% = rec("N_max_partecipanti") %></font></font></td>
<td width="23%"><font size="2" face="Verdana, Arial, Helvetica, sans-serif"><font color="#000099" size="1">Docente:<% = rec("docente") %></font></font></td>
</tr>
</table>


<% rec.MoveNext
Wend
rec.close%>