(I used 0.70.)
1. I erased the associate of .torrent by Explorer.
2. I deleted registory key "HKEY_CLASSES_ROOT\bittorrent" and "HKEY_LOCAL_MACHINE\SOFTWARE\Classes\bittorrent"
3. I executed BitComet.
# .torrent is associated to BitComet in this operation, but this is behavior for which I do not hope.
4. I associate to other Client(ex. abc)(Properties -> On the General tab, clicked Change).
# Under such a condition, the client set to double-click it starts.
# However, "HKEY_CLASSES_ROOT\bittorrent\shell\open\command" value was BitComet's path.
# "HKEY_LOCAL_MACHINE\SOFTWARE\Classes\bittorrent\shell\open\command" was also the same.
# But, a new value was made for "HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.torrent\Application".
# This value was new client's path.
5. I restarted BitComet.
# "HKEY_CURRENT_USER\~~\.torrent\Application" value had disappeared and .torrent file associated BitComet.
If I had changed 4 as follows, associate has not changed when BitComet is started.
4. I chenged "HKEY_CLASSES_ROOT\bittorrent\shell\open\command" value to new client path.
The problem was tentatively solved by this. Thank you.
But, If I don't check a "Default bittorrent client" options, I want to never associate it with BitComet.
Thank you for your correspondence.