Recently there has been someone on the internet acting like me and calling himself by my name. He chats to people on MSN Messenger and picks fights with people. These people come to me when they see me and are very mad at me. I try to tell them that it was not me but they just keep coming an I keep having to protect myself when they throw a punch at me. Is there something I can do? Is that a crime to act like someone else. Please help me.

Identity theft is a crime. But its not a crime to pretend to be someone else as long as you don't try to commit fraud. Unless this person has access to your passwords, he may use a similar screenname or address, but not the same. Tell your friends to look carefully at the name and tell them you only use these addresses (supply a list).

You could change your screename and IM/mail select people in your contacts to let them know that the previous one will no loner be valid. Explain why you have to do this.