Michel Gallant has posted a very nice article on MSDN about using PKCS#12 key with .NET.  PKCS#12 are keys, generally exported from certificates, that tend to be stored in .pfx or .p12 files.  Michel has provided some nice code for opening these files from within your managed programs, and using the keys stored in them for RSA encryption.  The code uses some interop with CAPI, so if you'd like to understand what he's doing a little better, I'd also recommend another one of his articles, on using CAPICOM and CAPI from .NET.