Disabling Python's emulated InterlockedCompareExchange (for Win95 
compatibility) cures the problem, so the underlying question is why the 
existence of InterlockedCompareExchange is not being autodetected on 64 
bit systems -- and that is apparently because GetProcAddress
(kernel,"InterlockedCompareExchange") returns NULL -- which makes sense 
since InterlockedCompareExchange appears to be implemented using macros 
instead of being served up through kernel32.dll.

So is Win95 still a supported platform?  

If not, then perhaps InterlockedCompareExchange emulation can simply be 
deleted.  

If so, then either some other approach needs to be adopted to activate 
emulation, or the emulated code needs to be fixed to behave like the 
native windows functions (which appear to only operate on the lowest 32 
bits).