In which case you should be able to either:
a) leverage CDO for Windows 2000 (or CDO for Exchange 2000), and use the GetEncodedContent method of the BodyPart property...; or
b) Install CAPICOM 2.0 (available from MS, and redistributable), a wrapper for the CryptoAPI which provides a Utilities object that allows Base64 encoding and decoding
For XP only (and W2003)you can use CryptoAPI's CryptBinaryToString and CryptStringToBinary functions