In BitTorrent, everyone doesn't download pieces of a file accordingly. Instead, it depends on the peers you're connected to and your client will download what's available among them. So, it's something random. That's why even though you're 80% done, you might not have some pieces that the 30% has. Therefore, your client will download from him. Hope it helps. Thanks.

Go to 'Options' and select 'Preferences', at the 'Queueing' tab, change the 'Maximum number of active downloads' value to '1'.

Try running uTorrent as administrator. You should be able to associate torrent files with it.