How to store passwords in Winforms application?

I have some code like this in a winforms app I was writing to query a user's mail box Storage Quota.

DirectoryEntry mbstore = new DirectoryEntry(
      @"LDAP://" + strhome, 
      m_serviceaccount, 
      [m_pwd], 
      AuthenticationTypes.Secure);

No matter what approach I tried (like SecureString), I am easily able to see the password (m_pwd) either using Reflector or using strings tab of Process Explorer for the executable.

I know I could put this code on the server or tighten up the security using mechanisms like delegation and giving only the required privileges to the service account.

Can somebody suggest a reasonably secure way to store the password in the local application without revealing the password to hackers?

Hashing is not possible since I need to know the exact password (not just the hash for matching purpose). Encryption/Decryption mechanisms are not working since they are machine dependent.