Private key storage

[shumaker]

We would like to have a private key for all our programs and dlls, so
that they can communicate encrypted data between one another. Alot of
the examples I see create a key from a hard coded byte array, but I've
read that storing keys within a program is bad. The communication
between components would be limited to a particular machine, I.e. I
want my Program A to make calls to DLL B, which would return encrypted
data which would be decrypted by A, but prevent some other program from
calling my DLL and being able to interpret the data returned.

How should this key should be stored?

I've also read things talking about encrypting a key using a
passphrase, which seems to me that the passphrase is just another key,
so the question arises again... How do you store that second private
key?