There are other errors in your page as well... I will attempt to explain them to you. I am using the validator at http://htmlhelp.com/tools/validator/ to do this.
First of all, convert all <br> to <br />. That will eliminate a lot of your errors.
The first error I get is one to do with your first list. You have:
<div class="boxbottom">
<ul>
<li><a href="index.htm" title="Home">/Home\</a></li><br>
<strong>Guides</strong><br>
<li> <a href="skills.htm" title="Skills">/Skill Guides\</a></li><br>
</div>
You can not have <br> tags inside a list. That kind of defeats the purpose of a list. Each new set of <li></li> automatically moves to the next line. You also haven't closed the <ul> tag. For this code, I would suggest using this instead:
<div class="boxbottom">
<ul>
<li><a href="index.htm" title="Home">/Home\</a></li>
</ul>
<strong>Guides</strong><br />
<ul>
<li> <a href="skills.htm" title="Skills">/Skill Guides\</a></li>
</ul>
</div>
The only other errors I get are because you forgot to close the <div> tag for the <div id="wrapper">, <div id="body">, and <div id="content">. To fix that, just add 3 </div> tags right before the </body> tag. That will validate your page as far as XHTML goes.
If you want to validate your CSS, use this validator:
http://htmlhelp.com/tools/csscheck/