Pidgin stores the buddy list, pounces, preferences, and other data as XML files in a “hidden” directory in Windows. This data can be useful for transferring data between Pidgin clients without reentering your data each time.
To copy/backup your data, head over to C:\Documents and Settings\[your username]\Application Data\.purple\ and copy the XML files.
To transfer them to a Pidgin Portable installation, paste the files in \PidginPortable\Data\settings\.purple\ overwriting any existing files.
Pidgin also stores cached buddy icons in the icons folder of the same directory and HTML logs (if enabled) in the logs folder (which can also be viewed through the main window with Buddies → View User Log.