Need software for loan payments with additional principal payment

[Adam Brand]

Basically my mother is loaning me 37k at 2percent. We set the term for 10 years but I'd really like to pay this off asap. I need something that can easily add additional payment to principal.  Any help would be greatly appreciated but this is for only one deal and would hope its not too pricey.  Anyone know of anything that can do the math for us and keep track for life of loan?  Thanks in advance.