Here is an article you may find helpful to your research:
 
What Makes a Banned Book?
http://www.education.com/magazine/article/banned_books/
 
As for Charlotte's Web, according to Wikipedia, Publishers Weekly listed the book as the best-selling children's paperback of all time as of 2000.
 
To find out why some may have banned this popular book, you might contact the American Library Association, which organizes Banned Books Week in September/October:
http://www.ala.org/ala/aboutala/offices/oif/bannedbooksweek/bannedbooksweek.cfm
 
Good luck on your project, and let us know what you learn!