I had to add some nonsense to my webpage that would tell IIS to download the
file and not try to process

private void DownloadFile(string virtualPath)
{
string filePath = Server.MapPath(virtualPath);
FileInfo targetFile = new FileInfo(filePath);
// clear the current output content from the buffer
Response.Clear();
// add the header that specifies the default filename for the
download/saveas dialog
Response.AddHeader("Content-Disposition","attachment; filename=" +
targetFile.Name);
// add the header that specifies the file size so that the browser can show
the progress
Response.AddHeader("Content_Length",targetFile.Length.ToString());
// specify that th eresponse is a stream that cannot be read by the client
and must be downloaded
Response.ContentType = "application/octet-stream";
// send the file stream to the client
Response.WriteFile(targetFile.FullName);
// stop the execution of this page
Response.End();
}

hope this helps