basta dare come ContentType application/vnd.ms-excel e poi "scrivi" una tabella
ti posto un esempio
codice:
<%@ Language=VBScript %>
<%
Dim FileName
FileName="test.xls"
response.buffer = true
response.ContentType = "application/vnd.ms-excel"
response.AddHeader "content-disposition", "inline; filename=" & FileName
response.write "<table border=1>"
response.write "<tr>"
for i = 1 to 15 step 3
response.write "<td>"
response.write i * i
response.write "</td>"
next
response.write "<td width=40>=SOMMA(A1:E1)</td>"
response.write "</tr>"
response.write "</table>"
response.flush
response.end
%>