Everyone always underestimates Hillary. Even against McCain she is only 2 points down according to most polls.
One thing people don't seem to get is that the Clintons are political geniuses. They don't make the kind of mistakes that Kerry did. They'll make sure Hillary says the right things, does the right things, and is presented as best as possible. Nope, no errors like the swift boat controversy, or the Mary Cheney incident.
And don't forget: an actual vision for the future rather than Kerry's "I'm not Bush".