Actually, in the United States, intellectual property rights are automatically assigned to the author/owners of the software even without registering the copyright. If you add to your code, or somewhere in your program, "All Rights Reserved by", then your name and the year, you've legally established your rights with the code. Some states grant further rights, and the federal ones are more complex and involved than that, but basically if you claim it's yours, and can back it up with original source code, you don't need to do anything else.
To back it up in court, though, you'd need to prove damages to a judge or jury, and for small amounts, it'd cost more than it'd be worth. However, usually a letter from the originator/owner of the code or a lawyer representing that person is sufficient to someone violating intellectual property rights to stop them.