AFAIK the main thing is to deny that it was you that downloaded it, deny that you have the file on your computer and deny that you have infringed their copyright (or of course, just ignore it) - the company will then have to prove that you have, and continue to infringe their copyright, which is a lot trickier than just getting an IP match on a file.
There have been a couple of successful civil cases for copyright infringement regarding downloading music, but they have relied on the person admitting that they had the file on their computer and they had downloaded - damages were minimal but the firm sucessfully obtained an undertaking for the individual to cease their copyright infringing action - this is deemed a success so the solicitors costs followed.