This just randomly happened, and I don't know whats wrong. I've triple checked over all my code and couldn't find what was wrong. What I am working on here is for my client and I really need to get this fixed so I can finish the rest of it. So whatever help will be greatly appreciated

. Thanks in advance!
Here is what is happening Note its the blue box that is messing up
Here is my CSS Code
Code:
/* CSS Document */
* {
padding:0;
margin:0;
}
body {
background-color:#ebebeb;
}
#graphic-container {
margin-top:70px;
vertical-align:middle;
height:531px;
max-height:531px;
max-width:494px;
width:494px;
background-color:#fdeada;
background-image:url(../images/inside-background.png);
background-repeat:no-repeat;
background-position:center center;
border:4px solid #395e8a;
}
#red_btn {
background-image:url(../images/red_btn.png);
background-repeat:no-repeat;
height:168px;
width:171px;
margin-top:75px;
margin-left:70px;
float:left;
}
#green_btn {
background-image:url(../images/green_btn.PNG);
background-repeat:no-repeat;
height:180px;
width:168px;
margin-top:75px;
margin-right:70px;
float:right;
}
#purple_btn {
background-image:url(../images/purple_btn.PNG);
background-repeat:no-repeat;
height:168px;
width:173px;
margin-top:9px;
margin-left:70px;
float:left;
}
#blue_btn {
background-image:url(../images/blue_btn.PNG);
background-repeat:no-repeat;
width:180px;
height:168px;
margin-top:9px;
margin-right:70px;
float:right;
}
.red_btn_text {
float:left;
}
Here is my HTML Code
Code:
<!--
Script Written By: Kevin Gilbert
Programmer Contact Information: kevin@scriptingfire.com
-->
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<link rel="stylesheet" type="text/css" href="css/style.css" />
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Clickable Graphics Project</title>
</head>
<body>
<center>
<div id="graphic-container">
<!-- START Top 2 Button Container -->
<div id="top_btn_container">
<!-- START Red Button -->
<div id="red_btn">
</div>
<!-- END Red Button -->
<!-- START Green Button -->
<div id="green_btn">
</div>
<!-- END Green Button -->
</div>
<!-- END Top 2 Button Container -->
<!-- START Bottom 2 Button Container -->
<div id="bottom_btn_container">
<!-- START Purple Button -->
<div id="purple_btn">
</div>
<!-- END Purple Button -->
<!-- START Blue Button -->
<div id="blue_btn">
</div>
<!-- END Blue Button -->
</div>
<!-- END Bottom 2 Button Container -->
</div>
</center>
</body>
</html>