Checar se login existe no banco e dar um alert !

Amigos como eu faço para receber um alert caso o login já exista no banco.

<?

include "dbconfig.php";
$acao = $_GET['acao'];
$id = $_GET['id'];
$Inserir = $_POST['Inserir'];
$Alterar = $_POST['Alterar'];
$Excluir = $_GET['Excluir'];
$form_cliente = $_POST['form_cliente'];
$form_email = $_POST['form_email'];
$form_login = $_POST['form_login'];
$form_senha = $_POST['form_senha'];
$form_tel_res = $_POST['form_tel_res'];
$form_tel_com = $_POST['form_tel_com'];
$form_celular = $_POST['form_celular'];
$form_ativo = $_POST['form_ativo'];
$form_observacoes = $_POST['form_observacoes'];
if ($_SESSION[nome] == "" or $_SESSION[login] == "") {
 header("Location:index.php?conteudo=altera_dados_usuario");

} else {

 if ($Inserir == "ok") {
 mysql_query("INSERT INTO clientes (cliente,email,login,senha,tel_res,tel_com,celular,ativo,observacoes) VALUES ('$form_cliente','$form_email','$form_login',md5('$form_senha'),'$form_tel_res','$form_tel_com','$form_celular','$form_ativo','$form_observacoes')");
 //header( 'Location: index.php?conteudo=clientes' ) ;
 die("<script type='text/javascript'>window.top.location='index.php?conteudo=clientes';</script>");
 }
 if ($Alterar == "ok") {
 mysql_query("UPDATE clientes SET cliente='$form_cliente',email='$form_email',login='$form_login',senha=md5('$form_senha'),tel_res='$form_tel_res',tel_com='$form_tel_com',celular='$form_celular',ativo='$form_ativo',observacoes='$form_observacoes' WHERE id_cliente=$id");
 //header( 'Location.replace: index.php?conteudo=clientes' ) ;
 die("<script type='text/javascript'>window.top.location='index.php?conteudo=clientes';</script>");

 }
 if ($Excluir == "ok") {
 mysql_query("DELETE FROM clientes WHERE id_cliente=$id");
 mysql_query("DELETE FROM processos WHERE id_cliente=$id");
 //header( 'Location: index.php?conteudo=clientes' ) ;
 die("<script type='text/javascript'>window.top.location='index.php?conteudo=clientes';</script>");
 }
?>

<script language=javascript>

//MÁSCARA DE VALORES

function txtBoxFormat(objeto, sMask, evtKeyPress) {
var i, nCount, sValue, fldLen, mskLen,bolMask, sCod, nTecla;

if(document.all) { // Internet Explorer
nTecla = evtKeyPress.keyCode;
} else if(document.layers) { // Nestcape
nTecla = evtKeyPress.which;
} else {

nTecla = evtKeyPress.which;
if (nTecla == 8) {
return true;
}
}
sValue = objeto.value;

// Limpa todos os caracteres de formatação que
// já estiverem no campo.

sValue = sValue.toString().replace( "-", "" );
sValue = sValue.toString().replace( "-", "" );
sValue = sValue.toString().replace( ".", "" );
sValue = sValue.toString().replace( ".", "" );
sValue = sValue.toString().replace( "/", "" );
sValue = sValue.toString().replace( "/", "" );
sValue = sValue.toString().replace( ":", "" );
sValue = sValue.toString().replace( ":", "" );
sValue = sValue.toString().replace( "(", "" );
sValue = sValue.toString().replace( "(", "" );
sValue = sValue.toString().replace( ")", "" );
sValue = sValue.toString().replace( ")", "" );
sValue = sValue.toString().replace( " ", "" );
sValue = sValue.toString().replace( " ", "" );
fldLen = sValue.length;
mskLen = sMask.length;
i = 0;
nCount = 0;
sCod = "";
mskLen = fldLen;

while (i <= mskLen) {
bolMask = ((sMask.charAt(i) == "-") || (sMask.charAt(i) == ".") || (sMask.charAt(i) == "/") || (sMask.charAt(i) == ":"))
bolMask = bolMask || ((sMask.charAt(i) == "(") || (sMask.charAt(i) == ")") || (sMask.charAt(i) == " "))

if (bolMask) {
sCod += sMask.charAt(i);

mskLen++; }
else {

sCod += sValue.charAt(nCount);
nCount++;
}
i++;
}
objeto.value = sCod;

if (nTecla != 8) { // backspace
if (sMask.charAt(i-1) == "9") { // apenas números...
return ((nTecla > 47) && (nTecla < 58)); }
else { // qualque caracter...

return true;
}
}

else {

return true;
}
}
</script>
<script language="JavaScript">
function pergunta()

{

if (confirm('Confirma exclusao?')) { return true }
else { return false }
}
</script>

<script language="JavaScript">
function critica()

{

if (document.login.login.value == "")
{ alert("Campo Obrigatório: LOGIN"); document.login.login.focus(); return (false); }
if (document.login.senha.value == "")
{ alert("Campo Obrigatório: SENHA"); document.login.senha.focus(); return (false) }
return (true);
}
</script>

<script language="javascript" type="text/javascript">
function validar() {
var form_cliente = form1.form_cliente.value;
var form_tel_res = form1.form_tel_res.value;
var form_login = form1.form_login.value;
var form_senha = form1.form_senha.value;

if (form_cliente == "") {
alert('Preencha o campo Nome');
form1.form_cliente.focus();
return false;
}

if (form_tel_res == "") {
alert('Preencha o campo Telefone');
form1.form_tel_res.focus();
return false;
}

if (form_login == "") {
alert('Preencha o campo Login');
form1.form_login.focus();
return false;
}

if (form_senha == "") {
alert('Preencha o campo Senha');
form1.form_senha.focus();
return false;
}

}
</script>

<table width="750" class="table">
<tr valign="top">

<?

 if ($acao != "") {
 $sql = mysql_query("SELECT * FROM clientes WHERE id_cliente=$id");
?>

*Não é necessário preencher todos os campos.

<form action="<?= $PHP_SELF ?>" method="post" enctype="multipart/form-data" name="form1">
<table width="450" >
<tr>
<td align="right">Nome:</td>
<td>
<input name="form_cliente" type="text" value="<?= @mysql_result($sql, 0, cliente) ?>" size="35" maxlength="150">
</td>
</tr>
<tr>
<td align="right">E-mail:</td>
<td><input name="form_email" type="text" value="<?= @mysql_result($sql, 0, email) ?>" size="35" maxlength="100"></td>
</tr>
<tr>
<td align="right">Telefone:</td>
<td><input name="form_tel_res" onKeyPress="return txtBoxFormat(this, '(99) 9999-9999', event);" type="text" value="<?= @mysql_result($sql, 0, tel_res) ?>" size="20" maxlength="14" placeholder="11 0000-0000">
</td>
</tr>
<tr>
<td align="right">Telefone Comercial:</td>
<td><input name="form_tel_com" onKeyPress="return txtBoxFormat(this, '(99) 9999-9999', event);" type="text" value="<?= @mysql_result($sql, 0, tel_com) ?>" size="20" maxlength="14">
</td>
</tr>
<tr>
<td align="right">Celular:</td>
<td><input name="form_celular" onKeyPress="return txtBoxFormat(this, '(99) 9 9999-9999', event);" type="text" value="<?= @mysql_result($sql, 0, celular) ?>" size="20" maxlength="16">
</td>
</tr>
<tr>
<td align="right"> </td>
<td> </td>
</tr>

<!--

<tr>
<td align="right">Endereço:</td>
<td><input name="form_endereco" type="text" value="<?= @mysql_result($sql, 0, endereco) ?>" size="35" maxlength="80"></td>
</tr>
<tr>
<td align="right">Bairro:</td>
<td><input name="form_bairro" type="text" value="<?= @mysql_result($sql, 0, bairro) ?>" size="35" maxlength="50"></td>
</tr>
<tr>
<td align="right">Cidade:</td>
<td><input name="form_cidade" type="text" value="<?= @mysql_result($sql, 0, cidade) ?>" size="35" maxlength="40"></td>
</tr>

-->

<tr>
<td align="right">Ativo: </td>
<td><input name="form_ativo" type="text" value="<?= @mysql_result($sql, 0, ativo) ?>" size="4" maxlength="1" placeholder=" 0- Desativado 1- Ativado">
</td>
</tr>
<tr>
<td align="right">Observações:</td>
<td><textarea name="form_observacoes" cols="40" rows="5"><?= @mysql_result($sql, 0, observacoes) ?></textarea></td>
</tr>
<tr align="left">
<td colspan="2"> </td>
</tr>
<tr align="left">
<td colspan="2"><img src="imagens/24-security-key.png" width="24" height="24" align="absmiddle">
Usuário para acesso a internet.</td>
</tr>
<tr>
<td align="right"><strong>Login:</strong></td>
<td><input name="form_login" type="text" value="<?= @mysql_result($sql, 0, login) ?>" size="35" maxlength="30"></td>
</tr>
<tr>
<td align="right"><strong>Senha:</strong></td>
<td><input name="form_senha" type="password" value="<?= @mysql_result($sql, 0, senha) ?>" ><!--size="35"> maxlength="20"-->
</td>
</tr>
<tr align="left">
<td colspan="2"><font size="1">Comunique o usuário o (login e senha) acima para que ele possa acessar a internet.</font></td>
</tr>
<tr align="right">
<td colspan="2"><input type="hidden" name="id" value="<?= $id ?>"> <input type="hidden" name="<?= $acao ?>" value="ok">
<input type="submit" onclick="return validar()" class="button" name="Submit" value="<?= $acao ?>"></td>
</tr>
</table>
</form>
<p align="center"> </p>

<?
} else {

?>
<table class="table table-striped" width="99%" >
<tr >
<td><b>Nome</b></td>
<td><b>Login</b></td>
<td><b>Ativo</b></td>
<td><b>Telefone</b></td>
<!--<td><b>E-mail</b></td>-->
<td width="10%" colspan="3"><b>Ação</b></td>
</tr>

<?

 $sql = mysql_query("SELECT * FROM clientes ORDER BY cliente ASC");
 while ($coluna = mysql_fetch_array($sql)) {
?>

<td >
<?= $coluna[cliente] ?>
</td>
<td >
<?= $coluna[login] ?>
</td>
<td>
<?= $coluna[ativo] ?>
</td>
<td>
<?= $coluna[tel_res] ?>
</td>
<!--<td> <a href="mailto:<?= $coluna[?>">](mailto:)

[](mailto:)
-->
">**
">**
" onClick="return pergunta();">**
[](mailto:)

[](mailto:)
[](mailto:)
[](mailto:)
[](mailto:)
[](mailto:)
** cadastrar novo usuário
[](mailto:)
[](mailto:)
[](mailto:)
[](mailto:)