Editting a file programatically

Hey guys,
I am working on an application that needs to open a file that has only one line, encrypt the content, and save it.  I don't want to create a new file; just want to open the file, make my changes and save it.  I am using the objects "streamreader" and "streamwriter" to accomplish this.  The problem I'm having is that when I decrypt the file might be somewhat corrupted.  My encryption code is very stable and works great(we use it for all our applications).  I am thinking it could be the way I'm writing to the file that is creating the problem.  Here is the code snippet:

string sLine = string.empty;
string sEncrypt = string.empty;

// Read the line from the file and encrypt it
using(StreamReader myReader = new StreamReader(@"c:\test.txt"))
{
      while((sLine = myReader.ReadLine()) != null)
      {
          // encrypt the message
          sEncrypt = myComponent.Encrypt(sLine);
      }
}

// Write the encrypted line back to the file                  
using(StreamWriter myWriter = new StreamWriter(@"c:\test.txt"))
{
      myWriter.WriteLine(sEncrypt );
      myWriter.Flush();
      myWriter.Close();
}

Is there another way I can try to manipulate the files.  Any suggestions appreciated.