Home Button going down after adding Social Networks Icons (PHP CSS code)

Hi,

I wanted to add social networks icons to my website. The style is kinda custom so maybe I made a mistake somewhere.

Here is my style.css

Here is my Header.php

This is the code I use in header.php:

<div id="header_icons">
<ul><li><a href="http://www.facebook.com/dancetraxxrecords" target="_blank" alt="Facebook"><img src="/images/social_net/facebook.png" /></li></ul>
<li><a href="http://www.twitter.com/DTRAXXRecords" target="_blank" alt="Twitter"><img src="/images/social_net/twitter.png" /></li></ul>
<li><a href="http://www.youtube.com/dancetraxxrecords" target="_blank" alt="YouTube"><img src="/images/social_net/youtube.png" /></li></ul>
<li><a href="https://plus.google.com/110939277150791472117/" target="_blank" alt="Google+"><img src="/images/social_net/googleplus.png" /></li></ul>
<li><a href="http://www.lastfm.fr/label/Dancetraxx+Records" target="_blank" alt="Last.FM"><img src="/images/social_net/lastfm.png" /></li></ul>
<li><a href="http://www.hardstyle.com/dancetraxx-records" target="_blank" alt="Hardstyle.COM"><img src="/images/social_net/hardstylecom.png" /></li></ul>
</div>

And in Style.css:

#header_icons{
   float: right;
   margin-top: 0px;
   margin-right: 5px;
   width: 400px;

}

#header_icons li{
     float: right;
   margin: 2px 2px;
   display: inline ;

}

Can you guys please help me?

Here is my live site so you can see the problem:

http://www.dancetraxxrecords.com