found the answer here: http://forums.esri.com/Thread.asp?c=158&f=2276&t=262352

...If you have an ASP-button in your 9.3 .NET webapp that calls a Response.Write() for downloading files you have to place that control explicit in an UpdatePanel that defines a trigger to that ASP-button. See code below. ...

>Hi Rob
>
>after gooling theat command I've come up with the following:

>        Dim OutFile As New System.IO.FileInfo(ZipFileNameAndPath)
>        Response.Clear()
>        Response.ClearContent()
>        Response.ClearHeaders()
>        Response.AddHeader("content-disposition", "attachment;filename=" + ZipFileNameAndPath)
>        Response.AddHeader("Content-Length", OutFile.Length)
>        Response.ContentType = "application/pdf"
>        Response.TransmitFile(ZipFileNameAndPath, 0, OutFile.Length)
>        Response.Flush()
>        Response.Close()
>        Response.End()

>But I am getting the folowing error:

>The message received from the server could not be parsed.

>
>Any Ideas?
>
>Thanks!
>
>>If your zip file is stored in a location that the user has access to via the site, you can just redirect them to that file. Otherwise, try using Response.TransmitFile(): http://msdn.microsoft.com/en-us/library/12s31dhy%28VS.90%29.aspx