Confrontale come numeriche:

codice:
<select name="menu" onChange="window.location.href=this.options[this.selectedIndex].value;">
<%Do while not rs1.eof
id=rs1("id")
cat=request("tipo")
%>
<option value="sel_prodotti.asp?tipo=<%=rs1("id")%>"
<%if cint(id) = cint(cat)  then%>selected<%end if%> name="<%=rs1("categoria")%>">
<%=rs1("categoria")%>
</option>

<%
rs1.movenext
loop
%>
</select>">