If you've read the bittorrent website, the above is blatently obvious.
a .torrent, as shak said, is only filled with checksum and hash, to let teh torrent client know which file to download.
The tracker is where the clients say "I have this file, I'd like to get this and that" It also checks that the file people are trying to upload, i.e. give to you is identicle to the one that is being shared.
the file: if someone has edited the file in any way, the CRC check will not be the same, the hash will be different and the tracker will reject it. so unless the sub group that you got it from virused it (which wouldn't work for the majority of viruses) there can be no alteration.
If you got a recoded one, there wouldn't be that many sources for it. And it would be from a dodgy tracker, not one of the "official" ones, which I suggest you use.
I got my milky bar... ='D