Neilo is correct, the problem is being caused because your doctype tag does not include the URL to a DTD for the browser to use.
You probably shouldn't change to the XHTML doctype though, I dont think your page was created using XHTML markup standards.
Instead of the doctype tag Neilo suggested, use the full HTML 4.01 transnational doctype tag:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">