I can confirm the crash of python.exe when saving PNG files using matplotlib 0.98.5.3 under Python 2.5.4 and 2.6.2 (32 bit binaries from python.org) on Windows Vista 64 bit. Saving to a PDF file works. The crash occurs in _png.write_png(). When replacing the file _png.pyd with version 0.98.5.2 saving of PNG files works. There were no changes in the write_png function since 0.98.5.2.
from matplotlib import pylab
pylab.savefig("test.png") # crashes here