Depends on what range you are dealing with. If you can guarantee that that every character in the unicode string is an ASCII character, you can convert it by treating the unicode string as bytes, and copying every second byte.
Otherwise, you can't do it directly, as it depends on the codepage of the system you convert it on, shift-JIS encoding, etc. Windows itself offers functions for doing this, but I don't know of any third party dll that wraps around that.
