your current content is in a table. so if you put that table inside another table which is horizontally and vertically aligned it will work.
put this code just after your body tag. you might not need the center tag.
Code:
<table cellspacing="0" cellpadding="0" border=0 height="100%" width="100%"><tr><td align=middle valign=middle><center>
and then just close out the table at the end of your html