Wise Bread discusses how to use your lunch break to your best advantage. If you're planning on using your time to begin or advance a business endeavor: PLEASE, Please, please remember this!
DO NOT use your company's computer equipment--AT ALL!! You'll be jeopardizing your job.
DO file any and all Conflict of Interest paperwork your company may require--even if it seems obvious to you that there would be no conflict of interest. Get it in writing that the company a) does not have a cause for firing or reprimanding you and b) that they will not seek any sort of interest due to owning your professional capabilities or that you used any of their equipment.
Have a productive lunch break but do not jeopardize your bread and butter while trying to make enough money for jam. These are the concerns my previous job had--yours is going to be different so be sure to understand your policies and perhaps consult with a lawyer to assure you know where you stand.