Is a vote for Hillary a vote for McCain?

[labguy22]

Sorry to say that I have been absent from any politics for the majority of life, so I will not feign complete or even partial understanding.
Recently I have been seeing signs and bumperstickers that say "Write in Hillary". Last weekend while visiting Grandma, she stated she would write in Hillary when it was time to vote.
I don't wish to try and discourage someone's right to vote their conscience, but isn't writing in someone other than the candidates in fact taking away a vote from one?
I explained to Grandma, who clearly states she wants nothing to do with McCain, that if she writes in Hillary, she is in fact taking away a vote from Obama. Her response was, "Well I don't care, it's not like McCain will win anyway."
In such a close race, this idea and attitude is frightening to me.