Command line prompt broken on XP with Python 2.5 - help!

[Endless Story]

My last version of Python was 2.4, running smoothly on XP with path c:
\Python24 - no need even to include this path in PATH; everything
worked as it's supposed to at the command line.

Just installed Python 2.5, after uninstalling 2.4 (and also 2.3 which
had lingered). Now if I open a shell in Windows Python is not
available! Here are the symptoms:

- If I open a shell using "Command line here" with XP Powertools, then
enter "python" at the prompt, nothing happens. I don't get a message
that the command is not recognized, but neither do I get the Python
prompt. Instead the Windows prompt comes back. No messages, no Python,
no nothing.

- If I go so far as to specify the full path to the Python executable,
I do get the Python prompt and Python appears to work properly -
except that I can't exit with CTRL-Z.

- If I open a shell from the Start menu, e.g. Start > run "command",
then try entering "python", the shell simply blows up.

What is going on here? I have uninstalled and reinstalled Python 2.5,
to no avail. I have gone so far as to specify c:\Python25 (the install
path) in my PATH variable, but this makes no difference. Right now
Python is unusable to me from the command prompt, meaning all of my
automation scripts that I run at the command line are gone too.