It's interesting that Frontpage is producing code that Internet Explorer cannot use properly.
I would suggest using Google and search for "image rollovers" and find an alternative method that works for all browsers.
I would recommend that you switch guestbook scripts to something that offers better support for spam control.
You say you run Wordpress and are getting comment spam there too. Google for Spam Karma 2 and install it. Then say goodbye to comment spam on your blog.
There was a PHP upgrade made to the servers (in December) but it should not have any effect on your board as there are others running the same version.
Put up an "Updating" index.html page with no links to anywhere else on the site. Rename the index file for the site something else to test like index_new.html.