Question: It *appears* that specifying the `KEY_WOW64_64KEY` flag ([reference](http://msdn.microsoft.com/en-us/library/aa384129%28VS.85%29.aspx)) when accessing a registry key under 32-bit Windows XP has no effect - that is, no error is thrown, and the key is opened as if you hadn't had the flag set.

I know Windows 2000 throws an error when it encounters this flag.

I want to make sure my app is compatible with as many versions of windows (2k and later) as possible. 

Is there a Microsoft reference that specifies each version of Windows' behaviour for this flag? In particular, I'd like something that validates my assumption that it has no effect at all on post-2k 32-bit Windows.

Answer: 
I can't speak to Windows 2000 or XP, but I know that on Vista and above, KEY\_WOW64\_64KEY opens the registry key in the 64bit registry if it's a 64bit OS and the 32bit registry if it's a 32bit OS.
Also, you should usually be avoiding this key - WOW64 provides a pretty complete "illusion" to 32-bit apps; just write your app properly on 32-bit without this flag, and it will still work on WOW64. Don't try to use this flag (and other mechanisms) to be "64-bit compatible".