0.7.31 is unable to run GUI plugins, you need at least 0.7.32 for that. The behaviour you see after installing 0.7.31 is the same I saw when installing GUI plugins for it. That said, I don't understand why the plugin failed on 0.7.33 (you did install the icons, didn't you?), but if you delete the plugin file from Calibre's config directory (to be found in the application data folder inside the user profile folder on XP), Calibre should at least run normally.
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