<?php 
    $table = "inscrits"; 
    $sql = "SELECT * FROM " . $table. " where id=".$_GET['id']." order by NOM";
    $res = mysql_query($sql); 
		$nb = mysql_num_rows($res);
    $i = 0; 
	   while ($i < $nb)
	   {
	$viadeo = mysql_result($res, $i, "viadeo"); 
	if $viadeo!=""
{   
    echo '<a href="http://'.$viadeo.'" target="_blank"><img src="../images/viadeo.jpg" border="0" width="60"/></a>';     
}
else 
{
		 $i++; 
    } 
    ?>

et les parenthèses ?

<?php
    $table = "inscrits";
    $sql = "SELECT * FROM " . $table. " where id=".$_GET['id']." order by NOM";
    $res = mysql_query($sql);
    $nb = mysql_num_rows($res);
    $i = 0;
	
    while ($i < $nb)
       {
	  $viadeo = mysql_result($res, $i, "viadeo");
		if ($viadeo!="")
		    {  
		  	echo '<a href="http://'.$viadeo.'" target="_blank"><img src="../images/viadeo.jpg" border="0" width="60"/></a>';    
	 	    }
		$i++;
	}
    ?>

$table = "inscrits";
$sql = "SELECT * FROM " . $table. " where id=".$_GET['id']." order by NOM";
$res = mysql_query($sql);

while ($data = mysql_fetch_assoc($res))
{
    $viadeo = $data["viadeo"];
    if($viadeo)
    {  
        echo '<a href="http://'.$viadeo.'" target="_blank"><img src="../images/viadeo.jpg" border="0" width="60"/></a>';    
    }
}

SELECT * FROM " . $table. " WHERE id=".$_GET['id']." AND viadeo != '' ORDER BY nom

voir même faire ta requete avec ta condition

SELECT * FROM " . $table. " WHERE id=".$_GET['id']." AND viadeo != '' ORDER BY nom