use 'WX' ignored

Python 2.3.4 on XP , latest matplotlib

The use 'WX' command is being ignored and the examples always come up in Tkinter.
Funny thing is, before the latest XP install it used to work as advertized (same versions of everything).
Also, matplotlib.get_backend() reports correctly before and after the use command (tkagg before, wx after),
and the commandline -dWX works, though sometimes incredibly slowly (for "tale of two subplots").
I've tried the wxpython embedding program using 2 splitter windows in a recent mail thread, and that works, too.

Any ideas or ways to debug this ?

Ray Pasco