This question does not contain enough details to answer it fully.  In general, if you received the goods (the windows and installation), you are required to pay for them.  If you signed a contract that included interest but you did not notice that it included interest, you will probably have to pay the interest.  On the other hand, if the contractor added the interest later, you should not be responsible for the interest.  If the contractor intentionally misled you about the interest, you might avoid paying the interest.If the interest amount is significant enough, you should contact a lawyer.  The lawyer can review the documents and recommend a course of action.