From what you say about not losing your guests info I guess you have a back up of the old database. You should be able to use the backup to create and populate the tables in your new database.
They ended up putting the same post in about 10 threads. Wouldn't be so bad but their instructions only apply to Mozilla/Firefox (about 10% of internet users) and had nothing to do with most of the threads problems.
The admin loop happens when you upgrade your files from Advanced Guestbook 2.2 to 2.3.1. To fix it download www.carbonize.co.uk/AG/upgrade.zip to upgrade your database tables but you will lose pictures. The alternative is to either apply the patch yourself or goto www.carbonize.co.uk/AG and download the pre patched file.
By login loop do you mean it logs you in but as soon as you click a link it logs you out?
BTW the site you mention had three hacked entries one of which took you to a fake Yahoo login screen which i have now reported.
Search for T_LNUMBER and quickly. I'm not repeating my advice as it's all in there but basically you have been hacked and ata aguess the url.php file has been altered.
basically yes. Most pages are made in three sections
HEADER
CONTENT
FOOTER
Your header usually contains such things as your banner and menu. The footer is just to close all the HTML. The content is the bit that varies on every page and this is where you want the guestbook to be.