This folder serves as a common repository for application-specific data for current user in Windows operating system. A typical path used by BitComet is C:\Documents and Settings%userprofile%\Application Data\BitComet (Windows XP) or C:\Users%userprofile%\AppData\Roaming\BitComet (Windows Vista or later). (This folder is being used starting with v1.18)
BitComet.xml - Saves all options set in the Options dialog and information about visual details like column size etc.
Downloads.xml - Saves task list, containing abstract information of all BT, HTTP and FTP tasks.
torrents*.xml - Saves detailed information for each BT, HTTP and FTP task.
torrents*.torrent - Saves torrent files for each BT task, used when starting BT task downloading.
share\my_shares.xml - Saves the torrent list shared by user in Torrent Share of Torrent Exchange. (in v1.16 and earlier, torrent list in Torrent Archive is also stored in this file)
share*.torrent - Saves the shared torrent files in Torrent Share list.
share\peer_shares.xml - Saves the torrent link list from Torrent Collection of Torrent Exchange, when user has disabled "Clear Torrent Collection list at program exit".
archive\my_history.xml - Saves torrent file list for Torrent History. (new in v1.17+)
archive*.torrent - Saves torrent files in Torrent History list.
rules\dhtnodes.dat - Saves DHT network nodes.
cache\post_info.xml - Saves information about task comment posts, rating, snapshots, etc.
A typical installation path of BitComet is C:\Program Files\BitComet.
CRASH.DMP - Generated when BitComet crashed, supposedly send to developer by the user using CrashReport, in order to help fixing bugs.
fav\fav_*.xml - Saves Torrent Sites displayed in Favorites Bar of BitComet for each language.
lang\bitcomet-*.mo - Saves all the UI text strings for each language. All users can help translating in https://translations.launchpad.net/bitcomet.
scripts*.lua - Saves LUA scripts used for intelligent rename function of HTTP download.
tools*.* - Utility programs used by BitComet, including UPNP tool, VideoSnapshot tool, plugins for IE and Firefox.
plugin_emule*.* - eMule plugin for BitComet; a separately installed, optional plugin, used to download data from the ED2K Network to accelerate BitTorrent downloading.
Saves data of pieces where the file ends within a transferred piece, if the user is only downloading some, but not all of the files in a torrent. Refer to FAQ for more info.
The APP_DATA folder is used by BitComet since v1.18. A new option "system.use_app_data" has been added to the Advanced page of the Options dialog. Users can switch the configuration folder of BitComet using this option.
If installed in PROGRAM_FILES folder, the default configuration folder will be in APP_DATA; otherwise the default configuration folder will be the installation folder. The latter situation is useful when installing BitComet in a removable USB disk.
BitComet will seek and find the existing configuration files in their current installation((In Windows Vista or Windows 7 if BitComet was installed in the Program Files folder then by "installation folder" should be understood the \Users%userprofile%\AppData\Local\VirtualStore\Program Files\BitComet folder, as all configuration files will be found there. This is independent of BitComet and due to the data virtualization feature introduced in Vista and will happen to any application which tries to write its configuration files in Program Files or any other system folder, for that matter, while not using elevated privileges.)) folder, and their installation folder will continue being used as the configuration folder, no matter if it's the PROGRAM_FILES folder or not.
BitComet will try to seek and use the configuration files in the current installation folder at first. If not found, it'll try to search for them in the APP_DATA folder. If still not found, it'll create new configuration files as in a new installation, in the APP_DATA folder or installation folder.
Check the option "system.use_app_data" in the Advanced page of the Options dialog. If it's set on true, disable it (set it to false) to have all configuration files moved back to the installation folder. After that, reinstall the old version of BitComet, and it should work properly.