diff -r ebf522ef3b5a -r 77022e502321 Doc/using/windows.rst
--- a/Doc/using/windows.rst	Thu Sep 23 11:09:15 2010 +0200
+++ b/Doc/using/windows.rst	Fri Sep 24 23:59:40 2010 +0900
@@ -174,6 +174,53 @@
 :program:`regedit` (start it by typing "regedit" into :menuselection:`Start -->
 Run`).
 
+How sys.path is formed 
+^^^^^^^^^^^^^^^^^^^^^^
+This section describes the functionality, not the implementation (ie, the order
+in which these are actually fetched is different)
+
+   * An empty entry is added at the start, which corresponds
+     to the current directory. 
+
+   * If the :envvar:`PYTHONPATH` exists, its entries are added next. 
+
+   * If registry for `Application Paths` - that is, sub-keys
+     under the main PythonPath registry key - exists, These are added next (the
+     order of sub-key processing is undefined).  HKEY_CURRENT_USER is searched and
+     added first. HKEY_LOCAL_MACHINE is searched and added next.  (Note that all
+     known installers only use HKLM, so HKCU is typically empty) 
+
+   * If the :envvar:`PYTHONHOME` is set, we believe it as `Python Home`.
+     Otherwise, We regards path of our host .EXE as `Python Home` and 
+     locate our `landmark` (:file:`lib\\os.py`) and deduce our home.
+
+      - If we DO have a `Python Home`: The relevant sub-directories (Lib, plat-win,
+        etc) are based on the `Python Home`
+      - If we DO NOT have a `Python Home`, the core Python Path is loaded from the
+        registry.  This is the main PythonPath key, and both HKLM and HKCU are
+        combined to form the path
+
+   * If we cann't locate the Python Home, have not had a :envvar:`PYTHONPATH`
+     specified, and can't locate any Registry entries (ie, we have _nothing_
+     we can assume is a good path), a default path with relative entries is
+     used (eg. :file:`.\Lib;.\plat-win`, etc)
+
+The end result of all this is:
+
+   * When running :file:`python.exe`, or any other .exe in the main Python directory
+     (either an installed version, or directly from the PCbuild directory),
+     the core path is deduced, and the core paths in the registry are
+     ignored.  Other `Application Paths` in the registry are always read.
+
+   * When Python is hosted in another exe (different directory, embedded via
+     COM, etc), the Python Home will not be deduced, so the core path from
+     the registry is used.  Other `Application Paths` in the registry are
+     always read.
+
+   * If Python can't find its home and there is no registry (eg, frozen
+     exe, some very strange installation setup) you get a path with
+     some default, but relative, paths.
+   
 
 Executing scripts
 -----------------