All winapi functions use UCS2. The "ASCII" versions are just wrappers that implicitly convert from the current locale (latin1, ShiftJIS, BIG5, ...) to UCS2. Try passing some Japanese text to the ASCII version of a function on a system that's set to eg. German and see what happens...
Most Win32 API functions don't even take string parameters, so there's no separate ASCII or UCS-2 version of those.
Of the functions that take strings, the old ...A() versions should never be called anyway since they depend on the obsolete concept of code pages. The proper way to work with strings on Windows (IMHO) is to encode all string data to UTF-8, only encode/decode from/to UCS-2 when needed, and call the ...W() functions explicitely (don't use the global UNICODE define).
