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HarryT - if it happens again try right click on the calibre icon in the task bar and select Close Window. I can't remember if the XP taskbar has equivalent, in Win 7 the taskbar got a total revamp.
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HarryT &
theducks - did you try Title bar red-X and/or Alt+Space/Close
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kovid - has Qt4 changed in recent times, AFAIK XP hasn't had an update since April 8. Unless
theducks is running a Point of Sale XP - real or spoofed
BR