New Serpentine belt sqeals and smokes

My old serpentine belt broke on my 2007
Ford Taurus, it was old and had cracks all
in it, so I didn't think anything of it, but I
remember smelling a burning or smokey
smell before it broke. I put a new belt on it,
it is the right belt, and according to every
diagram I looked at, it's on correctly. I
checked to be sure the pulleys on
everything turned freely including the idler
and tensioner before I put 5he new belt
on. When I started the engine I heard it
squeal briefly but smoke was coming  
from the engine compartment and I knew
it was coming from the belt. What could
be causing this problem.