Q: What is the difference between RegOpenKeyExA and RegOpenKeyEx Could find MSDN documentation only on RegOpenKeyExA.

A: The function RegOpenKeyExA is the ANSI version of the RegOpenKeyEx, which is a wrapper to the function RegOpenKeyExW, the UTF-16 version.
In the context of the functions in question, the ANSI version requires the second parameter (i.e. the name of the registry subkey to be opened) to be of a LPCSTR type whereas the UTF-16 version requires the same parameter to be of a LPCWSTR type.
