Your template looks fine. Post the page that is throwing the error message. It sounds like the page you built off of the template is the only one with the problem, not the template itself.
The error message pops up when I select new from template and choose this template. It doesn't let me go any further so I can't create a file at all.
Now I see it. Open your template, go to line 152 and delete this block of code:
<body onLoad="MM_preloadImages('../assets/APTCO_PP2.gif','../assets/APTCO_ES2.gif','../assets/A PTCO_DP2.gif','../assets/APTCO_BF2.gif','../assets/APTCO_SP2.gif','../assets/APTCO_CS2.gif ')">
it's a repeat of the <body> tag and it's confusing your template.
You should also validate your template. I see multiple incorrect entries for XHTML. For example, image tags should be closed with a " />" (space-slash-angled bracket), not just ">".
Thanks very much! Interesting - when I deleted those lines, DW said it was in a locked section and changes would be discarded. Still I was able to save the changes and it works.
Dreamweaver has the ability to auto-correct code issues like the problem you just experienced.
Go to Dreamweaver > Preferences (or for PC, go to Edit > Preferences).
Category: Code Rewriting
Select the checkbox 'Fix invalidly nested and unclosed tags'
When you reopen your file, you should see an alertbox, where DW wants to correct some code. Since you're using the XHTML doctype, your images should have a single tag closing <img src="" /> instead of <img src="">.