The movie doesn't usually ruin the book for me. If I don't like the film, I may be annoyed with what they did to the plot and characters, but that just makes the book all the more special to me because nothing can outdo it. If I don't like the film, then I just dismiss it, and stick to the book.
Some films have enhanced my enjoyment of books, however, as they sometimes make it easier to visualise. One example is 'The Road'.