Does this help:
http://www.west-wind.com/weblog/posts/76293.aspx
Response.ContentType = "application/octet-stream";
Response.AppendHeader("Content-Disposition","attachment; filename=logfile.txt");
Response.TransmitFile( Server.MapPath("~/logfile.txt") );
Response.End();
Response.TransmitFile is the accepted way of sending large files, instead of Response.WriteFile.