Consult an intellectual property/copyright attorney. Even though you did not do it, that might not keep them from going after you because your computer was used for purposes of downloading the movie.
My firm handles a lot of these types of cases. Unfortunately, we haven't found the plaintiff's lawyers to be especially interested in whether they're suing the right person or not.
There probably isn't much you can do until you see whether you get served with a lawsuit. If you do, however, you will need to file an answer denying the allegations and provide your evidence that you were not involved in the activity. You may want to consider consulting an attorney to help you.
You can reach Harkess & Salter LLC at (303) 531-5380 or info@Harkess-Salter.com. Stephen Harkess is an attorney licensed in the state and federal courts of Colorado. This answer is for general information only and does not create an attorney client relationship between Stephen Harkess or Harkess & Salter LLC and any person. You should schedule a consultation with an attorney to discuss the specifics of your legal issues.
Seek out a Colorado attorney who practices copyright law immediately. A pejorative term that is often used to describe lawyers who do this sort of important civil defense work is "Copyright troll defense attorney."
He or she can write and file a Motion to Quash Subpoena for you. If he or she defends these types of lawsuits often, he or she will have a form subpoena and might be able to do this for you inexpensively. The arguments used to defeat these types of subpoenas are pretty standard. Also discuss with your lawyer whether you should provide a sworn affidavit to attach to your motion to quash.
If you cannot afford to hire a lawyer, then you might look to the internet for some sample motions to quash. Below is a link to a blog that provides a sample. I do not endorse or recommend the blog or the sample motion. I am not advising you or anyone else to use it. I am providing it for your educational use only.