Recently, thanks to Amaury Forgeot d'Arc and Michael Schneider, Windows became more of a first-class platform for PyPy's Python interpreter. Most RPython extension modules are now considered working (apart from some POSIX specific modules). Even CTypes now works on windows!
Next step would be to have better buildbot support for all supported platforms (Windows, Linux and OS X), so we can control and react to regressions quickly. (Buildbot is maintained by JP Calderone)