I'm using wxWidgets, so it's the Win32 API at the lowest layer. For accessibility I want to make it support the NVDA screen reader which I think would make it support other readers as well :)
I suggest you test with Narrator rather than NVDA. Disclosure: I work on the Narrator team at Microsoft. But that's not why I say this. The reason is that NVDA and JAWS have some ugly hacks that they can use to make some GUI implementations (particularly using Win32 with GDI for graphics) accessible even if they're not accessible by design. For details, do some searching on the term "off-screen model". Narrator doesn't have this, so if your product works with Narrator, you know it's really accessible.