the reason that this problem is caused is because your forums don't decrease in size by 80% all in every page. For example the minimum width on your cmps page is 790px, and if you decrease the page width more, you'll see that scrollbars will appear.
To fix the problem in the homepage, replace your header code with this,
Code:
<!-- logo -->
<a name="top"></a>
<table border="0" style="width:$stylevar[outertablewidth];" cellpadding="0" cellspacing="0" align="center">
<tr>
<td align="$stylevar[left]" <if condition="THIS_SCRIPT == 'index'">width="690"<else />width="790"</if> nowrap="nowrap"><a href="http://www.4g63hp.com/forums/cmps_index.php"><img src="http://www.4g63hp.com/forums/banner/random.php" border="0" alt="4G63HP.COM DSM And EVO Performance Forums" /></a></td>
</tr>
<code here> </if>
</table>
<!-- /logo -->
<!-- content table -->
$spacer_open
$_phpinclude_output
however, the homepage's min width is 790px, but your forums are 690px, so I added a little if condition that I hope will work,
just test it out and let me know if it worked or not.