JavaScript

<select name="swich" onChange="swich(this.value)">
   <option value="div1">Показать div1
   <option value="div2">Показать div2
   <option value="div3">Показать div3
</select>

<div id="div1" name="div1" style="display: none">DIV1</div>
<div id="div2" name="div2" style="display: none">DIV2</div>
<div id="div3" name="div3" style="display: none">DIV3</div>

function swich(val) {
   document.GetElementById(val).style.display='block';
}

document.getElementById(val).style.display='';

<select name="swich" onChange="swich(this.value)"> 
   <option value="div1">Показать div1 
   <option value="div2">Показать div2 
   <option value="div3">Показать div3 
</select> 
<div id="div1" name="div1" style="display: none">DIV1</div> 
<div id="div2" name="div2" style="display: none">DIV2</div> 
<div id="div3" name="div3" style="display: none">DIV3</div> 
<script type="text/javascript">
function swich(val) { 
   document.getElementById(val).style.display='block'; 
} 
</script>

var old_id = 'doc_dogovor';
document.getElementById(old_id).style.display='none';

var old_id = document.getElementById('old_show_div').value;
document.getElementById(old_id).style.display='none';

alert(old_id);

<script src='js/main.js'></script>
<form name="form" id="form">
<input type=hidden name="show_div" id="show_div">
<table align=center>
<tr>
  <td>
      <span id=adddocument>Выберите документ:</span>
  </td>
  <td>
      <select onChange="select_add_doc(this.value);" id="doc_switch" name="doc_switch">
            <option value='0' SELECTED>-=Выберите документ=-"
            <option value='doc_dogovor'>Договор
            <option value='doc_chertej'>Чертеж
            <option value='doc_prikaz'>Приказ
                  ...бла-бла-бла...
      </select>
   </td>
</table>

<div id="doc_dogovor" name="doc_dogovor" style="display: none;">
 ...........................
</div>

<div id="doc_chertej" name="doc_chertej" style="display: none;">
 ...........................
</div>

и т.д.

</forn>

function select_add_doc(val) {
var old_id = document.getElementById('show_div').value;
document.getElementById(old_id).style.display='none';
document.getElementById(val).style.display='block';
document.getElementById('show_div').value = val;
}

<frameset cols="130,*" border="1" id="mainframe" name="mainframe">
   <frame src="menu.html" name="a1" NORESIZE scrolling="no">
   <frameset rows="200,20,*" border="1" id="childframe" name="childframe">
        <frame src="top.html" name="a2" id="a2" NORESIZE>
        <frame src="list.html" name="a4" id="a4" NORESIZE>
        <frame src="info.html" name="a3" id="a3">
    </frameset>
</frameset>

<frameset cols="*" rows="200,*">
      <frame src="users.html" name="users" id="users">
      <frame src="us_info.html" name="us_info" id="us_info">
<frameset>