open your wordpress directory,
go to wp-admin folder,
then open admin-header.php.
on line 104, locate this
<img id="header-logo" src="../wp-includes/images/blank.gif" alt="" width="32" height="32" /> <h1 id="site-heading" <?php echo $title_class ?>><a href="<?php echo trailingslashit( get_bloginfo('url') ); ?>" title="<?php _e('Visit Site') ?>"><span id="site-title"><?php echo $blog_name ?></span> <em id="site-visit-button"><?php _e('Visit Site') ?></em></a></h1>
replace it with
<img id="header-logo" src="../wp-includes/images/blank.gif" alt="" width="32" height="32" /> <h1 id="site-heading" <?php echo $title_class ?>><a href="<?php echo trailingslashit( get_bloginfo('url') ); ?>" target="_blank" title="<?php _e('Visit Site') ?>"><span id="site-title"><?php echo $blog_name ?></span> <em id="site-visit-button"><?php _e('Visit Site') ?></em></a></h1>
i just added this target="_blank" to open your blog in a new window.
i hope this wont destroy your blog.. well, just make a copy of your untouch admin-header.php for a backup.