On the system i'm working with, we're currently using a COM component for encrypting and decrypting with RC4 which uses the Windows Cryptography API.

I would love to use the mentalis security object to do the job, but I'm not sure what to do about the IV and also how to make the mentalis object work with all of our existing encrypted data (which was all done with the windows cryptography). Has anyone done this before or have some tips?

The RC4 algorithm is a stream cipher, so it doesn't use an IV. However the IV property is present because the RC4 class inherits it from SymmetricAlgorithm.

The ARCFour class and the RC4CryptoServiceProvider class currently accept 'null' as the IV as well as a byte array of zero or one bytes. For optimal compatibility with other classes such as the CryptoStream class and others, you can use a one-byte array as the IV. The actual value of the byte doesn't matter -- it's completely ignored.