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Ed Addis - try running calibre in Safe Mode with no network, nothing in startup, and no shell extensions - assuming that's still doable on XP
You could also try giving calibre a dedicated temporary folder see ==>>
CALIBRE_TEMP_DIR, I think that was available before 2.0.
Another thing to try is Calibre Portable, it's self contained, hence less susceptible to interference from other programs.
BR