There are many HTML syntax errors:
http://htmlhelp.com/cgi-bin/validate.cgi?u...&spider=yesThe external style sheets seem very bloated. There's also some CSS between the HEAD and BODY sections (it should be in HEAD), but since it's inside Conditional Comments the validator ignores it.
You don't need to put every single list item inside its own list:
CODE
<!-- Navigation item -->
<ul>
<li><a href="index.html">Home</a></li>
</ul>
<!-- Navigation item -->
<ul>
<li><a href="services.html">Services</a></li>
</ul>
--in fact that defeats the purpose of lists.
The following seems very strange and unnecessary, not to mention it appears to result in invalid markup for IE6 (a table inside a link?):
CODE
<li><a href="#">Clients<!--[if IE 7]><!--></a><!--<![endif]-->
<!--[if lte IE 6]><table><tr><td><![endif]-->
<ul>
<li><a href="login.html">Login</a></li>
<li><a href="livechat.html">Live Chat</a></li>
</ul>
<!--[if lte IE 6]></td></tr></table></a><![endif]-->
</li>
Not sure if this is valid CC syntax either (the last ">" seems redundant):
CODE
<!--[if IE 7]><!-->
The semantic structure of the HTML can be improved:
* Why put the text "Links" in a H1 element?
* Why use a definition list for the menu? An unordered list seems a better choice.
* Why not use H2 elements for sub headers (styled with CSS if necessary) instead of
CODE
<p><b>About Maginn Online UK.</b></p>
* Why the BR elements between paragraphs?
You could also remove the fixed width of the layout, it shouldn't be too hard to make the content column resize to fit the user's window size.
The menu links "Home" and "Services" appear in two places, which seems redundant. At the same time "Services" and "Projects" go to the same page, which might confuse; ditto for "Link to us" and "Contact". Not sure what the link "Mobile" is about?

The forum link is broken.
http://www.maginnonline.co.uk/services.html uses inline frames, which is not recommended.