Just in case anyone (like me) spent ages trying to figure out why the "Proceed directly to checkout" header is a different font size than the other headings (which causes a problem with the graphic corners to the box), look at this line in \includes\languages\english\login.php
:
define('HEADING_CHECKOUT', '<font size="2">Proceed Directly to Checkout</font>');
Remove the <font> tags.
These shouldn't be in here anyway as they are display tags - should be in the .css file if needed.