I'm doing some work with a custom version of Python 2.6 on Windows, and would love to use emacs for this purpose. On more unixy platforms, I've never had any issue with the basic python-mode stuff, but now I'm encountering a problem when I try to use the M-x
apply: Spawning child process: invalid argument
You could try
C-u M-x run-python, which gives you the option of selecting the path of the python binary you want to run.
Alternatively, if you would like to start it using Elisp, that can be achieved by calling
(run-python "/usr/bin/python3") for python3, for example.