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The biggest problem is that VC.NET uses a new C library,
msvcr70[d].dll.

I have not fully studied all issues, but it appears that,
for purposes of Python, you cannot mix extensions, atleast
not in the general case. In particular, you cannot pass
FILE* between both C libraries. This is particularly
annoying, since MS has managed that struct _iobuf (aka FILE)
has identical layout in both compilers. Nevertheless, it
crashes in the following scenario:

VC7:fopen
VC: fputs

The problem is that fputs wants to lock the file. For that,
it tests whether the pointer comes from its own _iob
(_file.c:_lock_file). If the pointer comes from the _iob of
the other C library, it concludes that this must be a _FILEX
(which it isn't), and crashes :-(

So it appears that one *must* build extension modules with
the Visual Studio that also has built Python.