The installers should include the subset of Cygwin that is needed to run PostgreSQL. Native PostgreSQL is PostgreSQL compiled natively for Win32 instead of using the Cygwin emulation layer. 8.0 was the first version of PostgreSQL to have a native Win32 port.
The advantage to using one of the installers is that they include a compiled version of AOLserver and a version of OpenACS that they have been tested with. The versions of PostgreSQL and AOLserver they include are a few years old though.
I would suggest trying the installers first for anyone new to OpenACS on Win32. If you run into difficulties with the installer, or need the performance increase and bug fixes from newer versions of the pieces, switch to PostgreSQL 8.0 and a newer build of AOLserver.
