Debbie Carraway in IT at NCSU gives this analysis of an
apparent bug in the Python 2.4 installer for Windows.
She discovered this in the context of using the
installer for VPython, which looks for Python 2.4 in

  HKLM\Software\Python\PythonCore\2.4\InstallPath

which used to work with Python 2.3. Here is her report:

The problem is with the Python 2.4.1 installer. Python will
put the InstallPath registry setting (which indicates where
Python is installed)  under HKEY_CURRENT_USER instead of
HKEY_LOCAL_MACHINE under some circumstances:
 (1) if you are not a member of the Administrators group
 (2) if you choose "Install for Just Me" rather than
"Install for All Users",
 (3) if you automate the Python install using "msiexec.exe
/i python-2.4.1.msi /q". The /q (quiet) switch causes
different behavior than the defaults with a manual install
("msiexec.exe /i python-2.4.1.msi").

This is a mistake in the Python installer. The path to an
installed application always belongs under HKLM,
because the
path to installed software is a machine-specific
setting. If
the path is in HKCU, the InstallPath setting can follow a
user to a machine where Python is not installed.

Logged In: YES 
user_id=1188172

As far as I can see, newer installers (created with
bdist_wininst) will look in both registry keys for Python
versions.

So I think a new VPython installer should solve this problem.