Either explicitly delete it or set its expiration date to 0 and it will be deleted when the browser is closed.

Personally, I am against opening too many multiple windows but that is a matter of personal choice. I feel that using a cookie or storing the values at the server will serve the same purposes without being as intrusive to the user.

Wayne Luke - Sitepoint Moderator/Internet Media Developer
Digital Magician Studios - Making Magic with Web Applications