Car Payment Loan

Car Loan need help with payment plan?
I am buying a car and got the loan through my father-in-law. I need something to figure out my payments and adjusts the interest according to what I pay per month. Is their software I can buy or something on the net, or microsoft office. I have looked at bankrate and they give me my payment plan at a locked in payment per month. I need something that adjusts from month to month.
Microsoft has a template available through their site that has a payment calculator – all you have to do is enter the amount borrowed, interest rate, length of time, and start date of the loan – it’ll do the rest. I believe they even have one that will figure out your balance if you make additional principals payments.
Also, a good site is…
http://www.surveyland.org/jump.php?link=auto
Good Luck.