The best place to find scripts for your store is
www.hotscripts.com, you will find a whole load of guest book scripts on this page
http://www.hotscripts.com/search?q=guestbook&cat=All. It is far better to to install your own script than use one of the hosted solutions such as Bravenet. If you use a hosted solution you will have to endure adverts and their own branding. If you install the software on your own the server, the most you are likely to have to endure is a simple text link to the author in the footer of the page. Looking at what you've got already, I assume you have one of the Lycos eshop packages? If you can let us know what server package you have, we'll be able to advise what is and isn't suitable for you.