How to tell if you're running on windows?

[Roy Smith]

I'm using 2.5.1.  How can I tell if I'm running on windows?  The
obvious answer, platform.system(), gets complicated.  On the python
that comes with cygwin, it returns 'CYGWIN_NT-5.2-WOW64', but I've got
a native windows build of python where it returns 'Microsoft'.

The real problem I'm trying to solve is whether to build a LIBPATH
environment variable with ';' or ':' delimiting the entries.  On the
cygwin build, os.pathsep returns ':', which isn't really correct.  If
you use that, you end up building paths that look like c:foo:c:bar.
It should be c:foo;c:bar