@
garbanzo - FWIW
Windows can be reinstalled without losing installed 3rd party software etc, a search for 'upgrade in place' will find plenty of links.
Calibre keeps its global settings in the location that opens when you press the
Preferences->Miscellaneous->Open calibre configuration directory button. On Windows the default is "C:\Users\
(user-name)\AppData\Roaming\calibre". If you want to put it elsewhere use the CALIBRE_CONFIG_DIRECTORY
Environment Variable.
Your custom icon theme should be in the calibre configuration directory.
Per library settings are in the library database.
BR