the author of this app says:
This is a loan calculator for your Pocket PC. Enter the total price, the down payment, the loan term and the APR to determine your monthly payment.
You can also enter an interest rate and calculate what your down payment should be. Conversely, you can enter your down payment and calculate how much you can afford to spend.
This program can also determine how much extra you need to pay on a loan to pay it off early in a given amount of time.