Did you download the file from here? May be worth redownloading it again nd overwriting the guestbook files with the ones you just downloaded. You do not need to run the install file again.
If we are talking about Advanced guestbook then the file needs to be i nthe guestbook folder as to maintain the file structure. Also the page needs to be .php and not .htm.
Why did your host suggest you upgrade apart from not knwong what they are on about? Check for the base_url setting in config.inc.php file and set it as http://lcavfd.com/guestbook
Would it be possible for you to email me your gustbook link along with the admin username and password so I could look at the files. My email is at the bottom of this post.