How to make msi installation read encrypted param file ?

Hi,

I wrote some software (add-in for word) and an msi for installing it.
I have few (~100) different installation sites, each with ~300-500 users.
I want to have different parameters for each installtion. these are for INSTALL time not for run time.
So I wrote a param file that contains all the parameters.
However, I want to verify that no one except me changed the parameters file.
That leads for a need to have a encrypted parameters file (fixed symetric key).
My question is how to implment this on the msi ? How can I supply it an encrypted param file
that is decrypted in install time and "merged" into the installation.

Important - I prefer to work on Visual Studio 2005. If not possible, please mention another product (installShield?) that is a must have.

Thanks

danny