Hm, the installer forgot to clean up, leaving lots of files, so contrary to the dialog's final message the system had been modified.
If those files are third-party libraries, this confirms my previous post. It's not the 3.1.1 installer that forgot to clean up, but files from a previous 2.x installation.
-- Gabriel Genellina -- http://mail.python.org/mailman/listinfo/python-list
