Dici una cosa cosi? io ho messo i colori invece che i bordi

Codice PHP:
<html>
    <head>
        <title>prova</title>
        <style>
            #contenitore
            {
                width:700px;
            }
            #header
            {
                background:yellow;
                height:100px;
            }
            #sinistro
            {
                background:red;
                height:600px;
                float:left;
                width:100px;
            }
            #centrale
            {
                background:green;
                height:600px;
                float:left;
                width:500px;
            }
            #destro
            {
                background:red;
                height:600px;
                float:left;
                width:100px;
            }
            .sistema
            {
                clear:both;
            }
        </style>
    </head>
    <div id="contenitore">
        <div id="header"></div>
        <div id="sinistro"></div>
        <div id="centrale"></div>
        <div id="destro"></div>
        <div class="sistema"></div>
    </div>
</html>