char* sz = "tadaaa";
CreateProcessW(A2W(sz), ...);

const char* sz = "tadaaa";
const CString s( sz );
LPCWSTR ws = static_cast<LPCWSTR>( s ); //calling CString::operator (LPCWSTR)()const;

typedef std::basic_string<TCHAR> tstring

// Make any methods you control return tstring values. Thus, you could write:
tstring command = obj.getInstallationPath();
CreateProcess(x, command.c_str(), ...);

LPCWSTR string_to_LPCWSTR(const std::string& s)
{
    std::basic_ostringstream<WCHAR> buffer;
    buffer << s;
    return buffer.str().c_str();
}