Okay, got it. Since it was the first time I was using BT, I hadn't quite unserstood that bits of files are downloaded randomly.

So I thought, gee, this text file is a few bytes large, it's been downloaded already, so I should be able to read it...
Well, no, of course. It was just random garbage on my hard disk waiting to get filled by bittorrent.

Everything working now, and thank you for helping me understand :-)