Guest book is one of the essential part for any website, To add a guestbook on your drupal site you can use the guestbook module, the problem you would face is adding a captcha to this now captcha is also very essential to control spam now how to add captcha its very simple just execute the following query
INSERT INTO `captcha_points` ( `form_id` , `module` , `type` )
VALUES
( 'guestbook_form_entry_form', NULL , NULL );
and you are done now go to /admin/user/captcha on your site and enable captcha on guestbook this would look something like this

captcha tip
Great tip I was looking everywhere for this. It works awesome.
Thanks
That was quick!!
great tip, that was really quick to add the form for captcha, i tried to add it from the admin panel but it didn't worked and then I found your tip.
thanks, keep them coming
Info in your blog help me
Info in your blog help me with my project, whitch based on букмекеры and i realy like your post!
Thanks!
Your tip worked perfectly! No more SPAM on my guestbook!
Other Drupal guestbook
Please take a look at the project page of the Drupal Guestbook (DGB).
http://drupal.org/project/dgb
The new DGB module is a alternative to the older Guestbook module and offers many missing features of the Guestbook module.
To use CAPTCHA on DGB entry forms please read the hint in the README.txt of the module.
Furthermore the DGB module works with the Spam module if you are use the additional Drupal Guestbook Spam module.
Thanks.
The DGB maintainer
this is still in dev, is
this is still in dev, is there a option available by which we can import comments of the older guestbook module.