This Old Code

Process.Start Method (String, String, SecureString, String)

process has a way to take a secure string

ProcessStartInfo myProcess = 
   new ProcessStartInfo("my.exe");

myProcess.UserName = "...";
SecureString password = new SecureString();
password.AppendChar('1'); // repeat
myProcess.Password = password;
myProcess.UseShellExecute = false;
Process.Start(myProcess);

ClickOnce Deployment for Visual C++ Applications

Deploying MFC applications via ClickOnce

Easy-to-use resumable .NET file downloader

Using BITS

Background Intelligent Transfer Service (BITS)