Download the ISO you want from Microsoft's archive at Digital River. Why bother with a Torrent?
If it's the Product Key you mean, good luck with that. Most torrentz come with hinkey key gens guaranteed to ruin your day.
As was posted above, if you're really too tight fisted to buy a genuine, worry free license, Google is your friend. It's just a question of how much faffing around is involved and how much your time is worth.