ACS:Law (A law firm in the UK) has been sending out THOUSANDS of bogus 'pre-settlement' letters to people it believes infringed on the copyright of its clients. Some how they have managed to swindle over a million pounds from unsuspecting people in the UK, even though there is NO legal reason for anyone to either respond to or pay these 'fines.'
The practice is being cloned here in the US by a couple of different law firms. Please don't be taken in. Especially if you receive a questionnaire like this one. It's almost laughable how hard these guys are trying to scam money out of innocent people.
Let's be clear. These guys have absolutely NO proof that you downloaded anything illegally. There is no evidence of infringement on your part, or anyone else. The best thing to do is just ignore them, and they should just go away.
Torrent Freak has a great write up with a ton of links to try and educate people on this. I suggest everyone read it.