Check this topic n the replies in there
Different kinds of copyright infringement letters
Level One: These kind of copyright infringement letters are just written warnings sent to you via email to let you know that you might have someone that is downloading copyrighted material using your IP. They are just generic letters sent by an automation email sender by your ISP.
Level Two: These copyright infringement letters are very similar to the Level One letters except that they usually contain warnings to get you to cease further infringement, and warn you that your ISP may shut off your internet service if you continue to violate copyright laws.
Level Three: These letters are never sent via email. If you get a email letter that is claiming to be level three from an ISP then I would consider it to be a level one or two letter. At this stage your ISP may shut off your internet service at some point. I would recommend that you contact your ISP claim that the files in question were deleted, and that it will never happen again.
Level Four: At this point the letter is threatening to cut off your internet service off due to a breach of the TOS (Terms of Service) agreement with your ISP, or is explaining that your service has already been suspended due to the same. In many cases the letters will be sent by an Anti-P2P organization requesting a settlement of some sort regarding the copyright violations.
Level Five: At this level the copyright infringement letter is sent via a mail service, peace officer, or court appointed official. Usually the letter is in the form of a subpoena to either invite you to court and/or let you know that the litigation process has started. At this point its time to hire a lawyer, and fight it out.