Large String Manipulation

Question

I have a large Text File 25MB (An Email). Each Line is seperated by an '\r\n' I wish to delete the the last two lines and the first line. How do I do it in the most efficent manner. I do not want to to Read it intirely into memory. I also want to maximize efficency and avoid many passess. Since I will be doing this to many large files. Any Suggestions? Thank You Vladimir