The magic word is <center>
Code:
<title>MinervaPendragon.com - Guestbook</title></head>
<body bgcolor="#555555" background="index/grad_text2-2.jpg" link="blue" vlink="blue" alink="active_link" topmargin="0" marginheight="0">
<center>
<table width="100%" border="0" cellspacing="0" cellpadding="4">
If you leave that out there, the appearance should be the same on both pages. (or add it to the other pages to get the ".php-look")
You should change the bgcolor="000000" in the nav-table where you show the menu-links to the other pages. Black BG with darkblue text shows up very poorly.
On the preview-page you should give people a chance to skip the preview if they are on dial-up. Put in e.g. a "click here" to continue or "click there" to skip, in whatever way you want to solve that.
The music is neat.
Nice result for a first trial.