Es fácil, sería algo así:
Código:
<%
Const openForReading = 1
Const openForWriting = 2
Const openForAppending = 8
Dim objFSO, objFile
Dim objTextStream, strFileName
strFileName = Server.Mappath("miarchivo.txt")
Set objFSO = Server.CreateObject("Scripting.FileSystemObject")
Set objFile = objFSO.GetFile(strFileName)
Set objTextStream = objFile.OpenAsTextStream(openForAppending)
objTextStream.WriteLine campo1&","&campo2&","&campo3
objTextStream.Close
Set objTextStream = Nothing
Set objFile = Nothing
Set objFSO = Nothing
%>
Saludos