How to create a simple, secure survey application?

Suppose you are writing a survey application and would like to somehow guarantee results to be secure from user stand point. Put simply, i know what IP you came from but i want to make sure you sleep well at night knowing i know nothing of your responses. I can't store IP in raw form.

I do need to guarantee 1 thing though, that is that you answer questions once. So once your PC comes in with some data, i need to recognize that your PC already has responsed to the survey.

Any suggestions on how to best handle it?

Thanks -mac