Some additional info:
The popup script will be there when someone visits your page for the 1st time, and that would also starts a session (with cookie). When he refreshes the page, this persistant cookie shall inform your PHP/ ASP(?) server not to create popup script again.
Specific to this popup, I think the cookie will persist for 15 days. That means he/ she'll won't get a popup again until after about 2 weeks.