[TheLastSuperman]

Edit the header template and find:

				<vb:if condition="$show['registerbutton']">
				<li><a href="register.php{vb:raw session.sessionurl_q}" rel="nofollow">{vb:rawphrase register}</a></li>
				</vb:if>

Replace with:

		                <vb:if condition="$show['guest']">
				<vb:if condition="$show['registerbutton']">
				<li><a href="register.php{vb:raw session.sessionurl_q}" style="margin-top:-3px;" rel="nofollow">{vb:rawphrase register}</a></li>
				</vb:if>
		                </vb:if>

I'm not sure why it's showing... you can try replacing to fix but make sure you have not recently been fiddling around in the header template and made an accidental change or something along those lines.