matplotlib's __init__.py is apparently looking
at os.environ['HOME'], but the default XP setting
for that is '%USERPROFILE%', which then gets passed
(*without* expansion) on by get_home even though it's
expansion is needed. So import pylab does not work.
I don't have time to look at this now.
This is a common problem on Windows.
I'll post a solution I've found when I'm back home.