If she becomes President, it will take an act of Congress to take her off the job, even if she is found guilty.
It would be kinda funny, however. A President serving her presidential term from prison.

The moment she gets indicted, the House will immediately vote to impeach her. This includes all the Republicans, and enough Democrats. As of right now, a vote for Hillary Clinton is a vote for her VP. With evidence mounting, an indictment is inevitable. She won't be serving a Presidential term in a prison cell, let alone while she is indicted.