use sys.platform == "win32" (fixes #23)

sys.platform == "win32" instead of sys.platform.startswith("win")
because "win32" is returend even on 64 bit Windows, too

See details:
http://stackoverflow.com/questions/2144748/is-it-safe-to-use-sys-platform-win32-check-on-64-bit-python/2145582#2145582