Capturing Referring URL With Cookie?

I am using a third party shopping cart service written in php. I don't have access to the php code so I decided to use javascript to write a cookie. What I'm trying to do is have the cookie capture the referring URL and then keep it for a day and have it follow where ever the visitor goes in the shopping cart. Then when they check out, the referring URL will populate a form field and be returned as part of the form results. This way I can track where my visitors come from. My code is below and I'm adding it to the file designated as the head section. (The files I have access to must use some form of include but I can't use php in them - just html and javascript. I've tried.)

I have an alert in the code that suggests something to do as well as set the cookie. The alert comes up and the cookie is set but when you go to another page, the alert comes up again and another cookie is set. I just need to have it set once so when they fill out the order form, I'll know where they came from.

<SCRIPT LANGUAGE="JavaScript">
cookie_name = "dataCookie";
var referred;

[code]....

In the body I am reading it with a form field of

<SCRIPT LANGUAGE="javascript">
document.write("<FORM>")
document.write("Originally Referred by:")

[code]....