While trying to reduce the size of my C: drive backup, I found that Bittorrent had been keeping all of the .torrent files I had ever used for downloading since I installed the program eight years ago. They were in User/Appdata/Roaming/Bittorrent. I manually deleted all but the last year of those files, 20k files totalling 1.16Gb. That caused Bittorrent to corrupt unrecoverably, the program disappeared and when I tried to delete in Control Panel, it said it was already deleted. When I tried to reinstall, it got to 47% unpacked and crapped out. I don't understand what has happened here, and I don't see why:
a) Bittorrent keeps the .torrent files after a torrent is deleted
b) Bittorrent totally corrupts if you manually delete old and unneeded .torrent files.