anim_tk.py fails

In revisiting an old project, I tried the example anim_tk.py.
On two different WinXP machines, a "ghost" window appears (can see right
through it)
and the following:

···

------------------------------------
C:\Python23\Lib\site-packages\matplotlib\examples>python anim_tk.py
elapsed 10.7944733008
Fatal Python error: PyEval_RestoreThread: NULL tstate

This application has requested the Runtime to terminate it in an unusual
way.
Please contact the application's support team for more information.
-------------------------------------

please embarrass me by pointing out that I've forgotten to install
something obvious.

BTW, It's not possible to say thank you enough times.
matplotlab/ipython/scipy is just a dream for scientific work. I also
add Maxima and Vpython (and TeX/LaTeX/ConTeXt) to my soup, fwiw.

-gary