Decrypting S/MIME

I need to decrypt S/MIME in an application I'm making. Does anyone know an API that's fairly easy to use, or some other tool that does the job. What I need my program to do is check the validity of digital signatures encrypted with SMIME.

I've been digging into Microsoft's CryptoAPI, but it's very low level and difficult to use. I'd like a higher level API to use with Visual C++ or Visual Basic. Nothing wrong with low-level APIs, but I've got one week for this assignment to be finished.