protected System.Web.UI.HtmlControls.HtmlInputFile file;

in upload button click event handling function write this code

if (file.PostedFile !=null) //Checking for valid file
			{		
				/* FileName property gives the entire path but we need only file name so here substring is used. */

				string _fileName        = file.PostedFile.FileName.Substring(file.PostedFile.FileName.LastIndexOf("\\") + 1) ;
				string _fileType        = file.PostedFile.ContentType ;
				int     _fileSize       = file.PostedFile.ContentLength;

				/*Checking for the length of the file. If length is 0 then file is not uploaded.*/

				if (_fileSize<=0)
					Response.Write("File Uploading failed for" + _fileName );
				else
				{
					file.PostedFile.SaveAs(Server.MapPath(".\\" + _fileName));
					Response.Write( "Your File " + _fileName + " of Type " + _fileType + " and size " + _fileSize.ToString() + " bytes was uploaded successfully");
				}


			}		

	int IntLoop=0;
			
			//Iterating through the Request.Files collection
			
			for(IntLoop=0;IntLoop			{
				if (Request.Files[IntLoop] !=null) //Checking for valid file
				{		
					/* Since the FileName gives the entire path we use Substring function to rip of the filename.*/
					
					string StrFileName = Request.Files[IntLoop].FileName.Substring(Request.Files[IntLoop].FileName.LastIndexOf("\\") + 1) ;
					string StrFileType = Request.Files[IntLoop].ContentType ;
					int    IntFileSize = Request.Files[IntLoop].ContentLength;
					
					/*Checking for the file length. If length is 0 then file is not uploaded.*/
					
					if (IntFileSize <=0)
						Response.Write(" Uploading of file " + StrFileName + " failed. 
");
					else
					{
						/*Saving the file to the web server*/
						Request.Files[IntLoop].SaveAs(Server.MapPath(".\\" + StrFileName));
						Response.Write( "Your file " + StrFileName + " of type " + StrFileType + " and size " + IntFileSize.ToString() + " was uploaded successfully.
");
					}
				}
			}

string filename = TextBox1.Text; 
			if (filename != "") 
			{ 

				string path = Server.MapPath(filename); 

				System.IO.FileInfo file = new System.IO.FileInfo(path); 

				if (file.Exists) 

				{ 

					Response.Clear(); 

					Response.AddHeader("Content-Disposition", "attachment; filename=" + file.Name); 

					Response.AddHeader("Content-Length", file.Length.ToString()); 

					Response.ContentType = "application/octet-stream"; 

					Response.WriteFile(file.FullName); 

					Response.End(); 

				} 

				else 

				{ 

					Response.Write("This file does not exist."); 

				}

			}