Well i did a view source in your code...you can start by removing the "center" forhits line
HTML Code:
<table class="tborder" cellpadding="6" cellspacing="1" border="0" width="100%" align="center">
If you do a view source on the main page while logged out I believe it's line #497.
I believe that is your problem, because since that is the main forum table below the header, that may be centering everything under it.
I hope this bit on info helps.