Aqui les posteo el formulario...
<form action="control.php" method="POST">
<table align="center" width="225" cellspacing="2" cellpadding="2" border="0">
<tr>
<td colspan="2" align="center"
<?if ($_GET["errorusuario"]=="si"){?>
bgcolor=red><span style="color:ffffff"><b>Datos incorrectos</b></span>
<?}else{?>
bgcolor=#cccccc>Introduce tu clave de acceso
<?}?></td>
</tr>
<tr>
<td align="right">USUARIO:</td>
<td><input type="Text" name="usuario" size="8" maxlength="50"></td>
</tr>
<tr>
<td align="right">PASSWORD:</td>
<td><input type="password" name="contrasena" size="8" maxlength="50"></td>
</tr>
<tr>
<td colspan="2" align="center"><input type="Submit" value="ENTRAR"></td>
</tr>
</table>
</form>