The resetting happens, because Xolox finishes download a part of the file and then finds out that it does not match up with the other parts you already have. Agreed, that could be handled more intelligently (checking earlier, before the part is downloaded completely), but would you prefer a corrupted file that would be no use at all to you?

For the napster thing: well - you will NEVER get the same results with a p2p network as with a sever based network. Thats just a fact of life, I'm sorry. On the other hand - it is rather unlikely that gnutella will ever go down in hellish flames like napster has.