If a site does not validate then you are relying on the browser to overlook any mistakes and render the site correctly. Modern browsers make a very good stab at this but there are no guarantees, especially between different families of browsers such as IE and Firefox.
First, you should specify a Document Type Definition at the top of each page as this sets out the rules for how the page should be displayed. Since your site uses tables I would suggest HTML 4.0 transitional so your doctype would be:
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "click here">
click here for more on Doctypes.
The eary errors relating to <meta name="Microsoft Theme" etc are because there is a space in the name. Replace the space with a hyphen"-" and this will then validate. e.g. "Microsoft-Theme".
Errors relating to the <p> tag are caused by the <font> tag being specified outside of the paragraph tags. Move these inside the paragraph tags, not forgetting to move the corresponding </font> tags, and these errors will disappear also.
The one or two errors that remain should be self explanatory.