Well, the obvious full protection solution would be a VPN service like IPredator, which conceals your IP address.
The anti-P2P agencies work by recording your IP while you were participating in downloading a copyrighted material. You’re probably one of many other hundreds who may have happened to be downloading that torrent at that particular point, and who may have received such a letter.
They don’t really monitor your actions specifically. If you happened to be among those IPs you were served. There is a case known in US when they served a printer which was residing on some floor in a big company, with such a letter!
The best course of action would be to ignore the letter, for now and see if they pursue that any further.
As a piece of advice, you should try to stay away from “hot” new torrents, unless you get them from private trackers which are “closed” communities and therefore, much harder to penetrate by anti-P2P shills. If you’re not a member in such a private site, you should try and become one and you’ll rest at more peace thereafter.
You may try to also download and install Peer Guardian. It blocks connections to some of the known anti-P2P organizations’ IP addresses (the P2P, government and spyware lists) therefore it may protect you in some cases but by no means should you be fooled into thinking that you’re away for good from the evil eyes of MPAA, just by doing that.
It also may help on certain torrents on which that same type of anti-P2P organizations act as “poison” releasers (uploading corrupted pieces into the swarm) if their IPs are on those lists.
There are a lot of pro and con opinions on using it (as to its usefulness) so you should make up your own mind on that.
You also may consider alternative download sources such as Usenet, but then you’ll have a somewhat harder time finding what you seek (until you learn your way around in that jungle) and you may have to be prepared to cash out something for a Usenet subscription (free Usenet access offered by many ISPs in their Internet packs, many times leaves much to desire).
But as I said, the cheapest and easiest solution to circumvent the wolves praying on you now and to get a good night’s sleep would be a VPN subscription to a service such as IPredator, which reroutes all your P2P connections through their servers, encrypts the connection between you and their servers (so that your ISP can legally say that they can’t peek into your traffic) and which claims that they don’t keep any logs on the servers.
I haven’t used it myself but is seems like the most sensible choice at present time.
