General copyright question

Hi there, I have seen some scripts that are copyrighted and one can not modify or use it for purpose. There are many variations in the conditions. My questions are... 1) Are these scripts legally copyrighted. I mean do we need to register the script at some office or what? 2) If yes how can I make my script fall under copyright act. 3) Is it country dependent? Thank a ton for your time. -- Kevin Javia