I found a good guide article on MSDN today on this topic:

USER and GDI handles are sign extended 32b values To facilitate the porting, a decision has been made that these system handles should stay as 32b values, sign extended to 64b on the 64b platform. That is, the individual handle types are still based on the HANDLE type, which maps to void *, and so the size of the handle is the size of the pointer, i.e. 4 bytes on 32b and 8 bytes on 64b. However, the actual value of the handle on the 64b platform, (i.e. the meaningful bits), fits within the lower 32b, while the upper bits just carry the sign. This should make it easy to port the majority of the application code. Handling of the special values, like –1, should be fairly transparent. It also should agree nicely with all the cases where the handles had been remoted with the help of the IDL definitions from the public file wtypes.idl. However, care needs to be taken when remoting the handles was done via a DWORD, as the upper long should be properly sign extended on the 64b side. The app should use HandleToLong() and LongToHandle() macros (inline functions) to do the casting right. So, in general we can have the following situations: A handle like HWND, HMENU, HPALETTE, HBITMAP etc was sent as its own type (that is specified with HMENU, etc. as the type of the argument) – do nothing, ole32.dll code handles situation as appropriate A handle was sent as a DWORD – you can: leave it as such and cast the wire value to handle using LongToHandle() change the argument to long or LONG_PTR; this is possible in some cases only, When designing a new interface, in order of preference: use the types themselves, use context handles or use a LONG64 argument to hack through. Going back to specifics of porting the legacy code, the following system handles fall into the nice 32b compatible category. The USER handles: HWND, HMENU, HICON, HCURSOR, HDWP, HHOOK, HACCEL, HWINSTA, HDESK, HKL, HMONITOR, HWINEVENTHOOK. The GDI handles: HBITMAP, HPALETTE, HMETAFILE, HENHMETAFILE, HMETAFILEPICT, HBRUSH, HFONT, HDC, HRGN .