tracking visitors till registration?

Hi there, I am wondering if following scenario would be possible: - Create a unique referer id to make sure that the visitior has been refered by a particular person. (e.g. server.com?ref=20) - The visitor enters the site and browses around, but the ref=20 should not be stored inside the url, but in a coockie - If the visitor signs up and there is a referer id assign the person who refered it some points. The problem I see, is how to make sure that the refered id does not get lost and in case the person comes back another day still be able to asign the person who refered it the points. Has anybody a good suggestion on that? Thanx for any hint, Andy