For now the studio is only trying to find out who you are. Your ISP notified you in case you want to object to the subpoena. If you do, you should immediately contact a lawyer in the state where the case is pending -- which might not be the state where you live. Depending upon what court the case is in, you may be able to object anonymously.
When and if the studio gets your identifying information, it will probably demand money from you to settle its claim. If you can't persuade it that you're the wrong person and/or reach a settlement agreement, it will probably either add your name to an existing lawsuit or start a new one against you.
The studio might even report you to law enforcement, though it presumably will not use the threat of doing so as leverage in your negotiations. That, after all, would qualify as extortion in many states.
The studio will not be able to actually seize your computer. But once you are a party to its lawsuit, it will probably be able to make you let it examine the computer for evidence of the download. That examination will likely reveal any steps you might take in the meantime to cover your tracks, so don't act rashly.
I don't know offhand what arguments you might make against the subpoena. There might be several. The sooner you contact a lawyer, the better your chances will be.
Good luck.
