I have a friend that has recently started a small trucking company. He is looking for some accounting software that is low-cost or cost-free. I'm wondering if there might be an open-source solution out there?

He is a Windows user, so something for the Windows platform would be ideal, but he is familiar with Ubuntu so that is an option for him if need be.

Any suggestions, recommendations, testimonials or nudges in the right direction would be appreciated.