while checking my shared-folder i found a complete download of a japanese-porno and some incomplete ones. I ve never done such downloads. Where are they from??? PS: if someone is interested I would share it. hahaha

Yes you did download it although the file you chose to download probably had a completely innocent filename. While swarming the filename might change and you probably see now a more accurate filename in your shared directory.

This happens when downloading spoofed results. Gnutella's form of spam.