What *exactly* are they charging you for? An early termination fee? Failure to return their modem and any other equipment you may have rented from them? Have you paid all charges in full before the payment due date? Additionally, since you signed a contract that obligated you to WindStream somehow, just exactly what is it, that isn't your fault?
Carl's got some good questions there - but if you signed up for it originally, IMHO you've got to give them another call and figure things out. Depending upon the length of time past, you might need to call them, or if they've sent it to a collection agency call that agency on your credit report.
Good luck, let us know how it goes.