If you only have the entire file's hash and the one you downloaded has a different one you still have no way of knowing which part of the file has bad data. BitTorrent splits up files into much smaller pieces (such as 1MB each) and provides hashes for each one. If the file you have is bad all you'll know exactly which pieces are corrupt and only need to redownload these. The torrent I posted above also won't do any P2P transfers to save on bandwidth, it will download only from MS.