What is the difference between C and Posix locales on Postgres?
Note that PostgreSQL just uses the operating system locale, and doesn't have it's own definition of locales. If your system is POSIX compliant, C and POSIX will be the exact same locale: http://www.opengroup.org/onlinepubs/009695399/basedefs/xbd_chap07.html (see the first sentence under 7.2).
According to The Single UNIX ® Specification POSIX and C locales are aliases:
The strings "C" and "POSIX" are reserved as identifiers for the POSIX locale
So they are guaranteed to be the same on POSIX systems. I don't know how far Windows Vista or 7 are POSIX compliant, but probably similar definition can be found in Microsoft documentation.