Funziona, seppur con una piccola modifica. Infatti usando la classe mi funziona, ma non con l'attributo backgrouncolor. Qui la funzione modificata. Non male per un newbie, eh 
Grazie mille per l'aiuto!!!! 
codice:
function changecolor(me)
{
if(lastSelectedElement && lastSelectedColor)
lastSelectedElement.className = lastSelectedColor;
lastSelectedElement = me;
lastSelectedColor = me.className;
me.className = "selected";
}
L'intero codice invece è questo:
codice:
<!doctype html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<HTML> <HEAD>
<script language="Javascript">
var pars="";
var lastSelectedColor = "";
var lastSelectedElement = "";
function changecolor(me)
{
if(lastSelectedElement && lastSelectedColor)
lastSelectedElement.className = lastSelectedColor;
lastSelectedElement = me;
lastSelectedColor = me.className;
me.className = "selected";
}
function alternate(id){
if(document.getElementsByTagName){
var table = document.getElementById(id);
var rows = table.getElementsByTagName("tr");
for(i = 0; i < rows.length; i++){
//manipulate rows
if(i % 2 == 0){
rows[i].className = "even";
}else{
rows[i].className = "odd";
}
}
}
}
</SCRIPT>
<style>
.odd{background-color: white;}
.even{background-color: gray;}
.selected{background-color: red;}
</style>
</head>
<body onload="alternate('thetable')">
<table id="thetable">
<tr><td width="0%"></td>
<td>Legal entity enabling </td>
<td>Last name</td>
<td>E-mail address</td>
<td>Enabled</td>
</tr>
<tr id= "ID1" onclick="changecolor(this);" name ="AAA">
<td width="0%"><input type="hidden" name="USER_ID[0]" value="AAA" disable = "disabled"></td>
<td>d</td>
<td>d</td>
<td>cicici</td>
<td></td>
<td> <input type= 'checkbox' name='FLG_ROLE_CODE_EDIT'></td></tr>
<tr id= "ID2" onclick="changecolor(this);" name ="BBB">
<td width="0%"><input type="hidden" name="USER_ID[0]" value="BBB" disable = "disabled"></td>
<td>d</td>
<td>d</td>
<td>cicici</td>
<td></td>
<td> <input type= 'checkbox' name='FLG_ROLE_CODE_EDIT'></td></tr>
<tr id= "ID3" onclick="changecolor(this);" name ="BBB">
<td width="0%"><input type="hidden" name="USER_ID[0]" value="BBB" disable = "disabled"></td>
<td>d</td>
<td>d</td>
<td>cicici</td>
<td></td>
<td> <input type= 'checkbox' name='FLG_ROLE_CODE_EDIT'></td></tr>
</table>
</form>
</body>
</HTML>