Enumerating all subkeys and values in a Windows registry key

It appears that you are calling RegEnumValue() without setting the lpcchValueName parameter to a proper value. This parameter is an [in] parameter as well as an [out] parameter. Try this:

for (int i = 0; i < numValues; i++)
 {
  DWORD valNameLen = 64; //added this line to match valueName buffer size
  RegEnumValue(hKey,
     dwIndex,
     valueName,
     &valNameLen,
     NULL,
     &dataType,
     (BYTE*)&data,
     &dataSize);

Documentation for RegEnumValue() : http://msdn.microsoft.com/en-us/library/ms724865(v=vs.85).aspx

Enumerating the keys this way is overkill. This would simply waste the system resources, memory, call-stack and put pressure on registry sub-system. Do not do unless needed.

Are you going to have "search registry" in your application? If yes, enumerate only when user demands so. Or, if you are developing "Registry Viewer/Editor", do expand and open sub-keys only when needed.

If you absolutely need to retrieve and store all keys/values, you can use multiple threads to enumerate the keys. The number of threads would initially be the HKEY-major-keys, and then you can have more threads, depending on number of sub keys and runtime heuristics you perform while enumerating the keys.

Recursion may or may not be good approach for "recursive-enumeration" of sub-keys - you must keep number of arguments to recursive implementation minimum - put the arguments into one struct or put them in class. You may also like to use std::stack for the same.