System.IO.Stream iStream = null;

// Buffer to read 10K bytes in chunk:
byte[] buffer = new Byte[10240];

// Length of the file:
int length;

// Total bytes to read:
long dataToRead;

// Identify the file to download including its path.
string filepath = @"E:\software\SQL Server 2000 Personal Edition.ISO";

// Identify the file name.
string filename = System.IO.Path.GetFileName(filepath);

try
{
// Open the file.
iStream = new System.IO.FileStream(filepath, System.IO.FileMode.Open,
System.IO.FileAccess.Read,System.IO.FileShare.Read);
Response.Clear();

// Total bytes to read:
dataToRead = iStream.Length;

long p = 0;
if(Request.Headers["Range"]!=null)
{
Response.StatusCode = 206;
p = long.Parse( Request.Headers["Range"].Replace("bytes=","").Replace("-",""));
}
if(p != 0)
{
Response.AddHeader("Content-Range","bytes " + p.ToString() + "-" + ((long)(dataToRead - 1)).ToString() + "/" + dataToRead.ToString());
}
Response.AddHeader("Content-Length",((long)(dataToRead-p)).ToString());
Response.ContentType = "application/octet-stream";
Response.AddHeader("Content-Disposition", "attachment; filename=" + System.Web.HttpUtility.UrlEncode(Request.ContentEncoding.GetBytes(filename)));

iStream.Position = p;
dataToRead = dataToRead - p;
// Read the bytes.
while (dataToRead > 0)
{
// Verify that the client is connected.
if (Response.IsClientConnected)
{
// Read the data in buffer.
length = iStream.Read(buffer, 0, 10240);

// Write the data to the current output stream.
Response.OutputStream.Write(buffer, 0, length);

// Flush the data to the HTML output.
Response.Flush();