Curing Serana is a pretty simple matter. It consists of never, ever confronting her or questioning her beliefs. The basic Serana dialog tree looks something like this: Serana: "I believe that the sky is green." Responces: 1: "I absolutely agree, Serana" 2: "I agree, Serana"" 3: "You're probably right, Serana." You'll want to go with number one. The most flattering and last confrontational of the flattering and non-confrontational options. If you dare challenge Serana at any point (suggesting that she cure her "condition". Gasp! You monster! You're racist against people that ..eat..people!) she'll clam up and the topic will be closed for discussion forever. But, if you turn your brain off and mindlessly agree with everything she says she will eventually be open to the idea of a cure after Harkon has been defeated. The secret is that you are actually Serana's follower.