Storing service account credentials securely in cl

I'm writing a ClickOnce application that runs a batch file process with service account credentials. I need to store the service account credentials so that the program can add the username/password to the process.startinfo property before running the process. The users do not know this password, so there's no prompt for them to enter in a password. I believe this means I cannot store the hash and verify the password that way, the hash value I generate must be reversible so that it can add the correct password to the startinfo property. I searched around this site and came up with a Frankenstein-type solution that works, but it's not very secure. Currently, I used this method to encrypt the password, stored the encrypted value, then use the decrypt method to obtain the password during runtime (the encrypt method is never ran during runtime, I ran it in Visual Studio during debug, copied the value, then used that value in the decrypt method below this):

// used to generate decrypted acct creds
    private void EncryptText(string plaintext)
    {
        string outsrt = null;
        RijndaelManaged aesAlg = null;

        try
        {
            // generate key from secret and salt
            Rfc2898DeriveBytes key = new Rfc2898DeriveBytes(sharedsecret, _salt);

            aesAlg = new RijndaelManaged();
            aesAlg.Key = key.GetBytes(aesAlg.KeySize / 8);

            ICryptoTransform encryptor = aesAlg.CreateEncryptor(aesAlg.Key, aesAlg.IV);

            using (MemoryStream mEncrypt = new MemoryStream())
            {
                // prepend the IV
                mEncrypt.Write(BitConverter.GetBytes(aesAlg.IV.Length), 0, sizeof(int));
                mEncrypt.Write(aesAlg.IV, 0, aesAlg.IV.Length);
                using (CryptoStream csEncrypt = new CryptoStream(mEncrypt, encryptor, CryptoStreamMode.Write))
                {
                    using (StreamWriter swEncrypt = new StreamWriter(csEncrypt))
                    {
                        // write all data to the stream
                        swEncrypt.Write(plaintext);
                    }
                }

                outsrt = Convert.ToBase64String(mEncrypt.ToArray());
            }
        }
        finally
        {
            if (aesAlg != null)
                aesAlg.Clear();
        }

        Console.WriteLine(outsrt);
    }

Here's the decrypt method:

private string GetServiceAcctPW()
    {

        // Declare the RijndaelManaged object
        // used to decrypt the data.
        RijndaelManaged aesAlg = null;

        // Declare the string used to hold
        // the decrypted text.
        string plaintext = null;

        try
        {
            // generate the key from the shared secret and the salt
            Rfc2898DeriveBytes key = new Rfc2898DeriveBytes(sharedsecret, _salt);

            // Create the streams used for decryption.                
            byte[] bytes = Convert.FromBase64String("EncryptedValueHere");
            using (MemoryStream msDecrypt = new MemoryStream(bytes))
            {
                // Create a RijndaelManaged object
                // with the specified key and IV.
                aesAlg = new RijndaelManaged();
                aesAlg.Key = key.GetBytes(aesAlg.KeySize / 8);
                // Get the initialization vector from the encrypted stream
                aesAlg.IV = ReadByteArray(msDecrypt);
                // Create a decrytor to perform the stream transform.
                ICryptoTransform decryptor = aesAlg.CreateDecryptor(aesAlg.Key, aesAlg.IV);
                using (CryptoStream csDecrypt = new CryptoStream(msDecrypt, decryptor, CryptoStreamMode.Read))
                {
                    using (StreamReader srDecrypt = new StreamReader(csDecrypt))

                        // Read the decrypted bytes from the decrypting stream
                        // and place them in a string.
                        plaintext = srDecrypt.ReadToEnd();
                }
            }
        }
        catch(Exception e)
        {
            Console.WriteLine("Error decrypting password");
            Console.WriteLine(e.StackTrace);
            logger.WriteToLog(Logger.LogCodes.ERROR, "Error decrypting service account password");
            MessageBox.Show("An error occurred while trying to start the installation process\nPlease contact the Service Desk for further assistance");
        }
        finally
        {
            // Clear the RijndaelManaged object.
            if (aesAlg != null)
                aesAlg.Clear();
        }

        return plaintext;
    }

This code works just fine, however, I know it's not secure. My code review guy said he was able to crack it with dotPeek in an hour because it's only adding a layer of obfuscation. What would be the best/proper way to store these credentials within the application?

The encryption key is on a dedicated server.

The password is sent to the server along with an id to be encrypted and the encrypted password returned for DB storage.

When the the password is needed a request is made to the dedicated server with the id and a decrypted password is returned.

The password is never saved to disk and the key is never available off the dedicated server.

The dedicated server is kind-of-like a poor-mans HSM.

This is encryption, not hashing. The encryption key is secret along with a random IV that that is saved with the id on the dedicated server. The key is not available and not related to the password so there is no better attack than brute force against the encryption key which is essentially to large to be attacked by brute force.

The server needs to be very secure, only a couple of two factor logins and not available to the Internet.