thanks got it fixed:
if (!empty($_POST['UserName']) && !empty($_POST['Password'])) {
$UserName = $_POST['UserName'];
$Password = $_POST['Password'];
} *<---was missing a curly bracket..oi syntax*
$msg = '';
if (!empty($UserName)) {
$sql = "SELECT `AdminID`,`UserName` FROM `admin` WHERE
`UserName`='$UserName' and `Password`='$Password'";
$result = mysql_query ($sql);
$row = mysql_fetch_object ($result);
If (mysql_num_rows($result) > 0) {
$_SESSION['AdminLogin'] = true;
$_SESSION['user'] = $UserName;
$_SESSION['AdminID'] = $row->AdminID;
header ('Location: Main.php');
exit;