//Checks if there is a login cookie
if(isset($_COOKIE['SecureID']))

//if there is, it logs you in and directes you to the members page
{
$username = $_COOKIE['SecureID'];
$pass = $_COOKIE['Password'];
$check = mysql_query("SELECT * FROM Admin WHERE AdminID = '$username'")or die(mysql_error());
while($info = mysql_fetch_array($check))
{
if ($pass != $info['Password'])
{
}
else
{
header("Location: WEBSITE");

}
}
}

//if the login form is submitted
if (isset($_POST['submit'])) { // if form has been submitted

// makes sure they filled it in
if(!$_POST['SecureID'] | !$_POST['Password']) {
die('You did not fill in a required field.');
}
// checks it against the database

$check = mysql_query("SELECT * FROM Admin WHERE AdminID = '".$_POST['SecureID']."'")or die(mysql_error());

//Gives error if user dosen't exist
$check2 = mysql_num_rows($check);
if ($check2 == 0) {
die('Incorrect ID.');
}
while($info = mysql_fetch_array($check))
{
$_POST['Password'] = stripslashes($_POST['Password']);
$info['Password'] = stripslashes($info['Password']);

//gives error if the password is wrong
if ($_POST['Password'] != $info['Password']) {
die('Incorrect password, please try again.');
}
else
{

// if login is ok then we add a cookie
$_POST['SecureID'] = stripslashes($_POST['SecureID']);
setcookie(SecureID, $_POST['SecureID']);
setcookie(Password, $_POST['Password']);

//then redirect them to the members area
header("Location: WEBSITE");
}
}
}
else
{

// if they are not logged in
?>
<form action="<?php echo $_SERVER['PHP_SELF']?>" method="post">
<table border="0">
<tr><td colspan=2><h1>Login</h1></td></tr>
<tr><td>Username:</td><td>
<input type="text" name="SecureID" maxlength="40">
</td></tr>
<tr><td>Password:</td><td>
<input type="password" name="Password" maxlength="50">
</td></tr>
<tr><td colspan="2" align="right">
<input type="submit" name="submit" value="Login">
</td></tr>
</table>
</form>
<?php
}

?>

// Should be using a function like isset() and double pipes for OR... ||
if(!$_POST['SecureID'] | !$_POST['Password']) {
die('You did not fill in a required field.');
}

while($info = mysql_fetch_array($check))

while($info = mysql_fetch_array($check, MYSQL_ASSOC))

Or better

while($info = mysql_fetch_assoc($check))