Put all the checks into a function. You might need to declare the $register_* variables global if you intend to use them later. I don't like setting and testing lots of flags, that's usually a sign of bad design.
This is untested but should give you the idea. You should have additional checks for maximum string length and invalid characters on every field (don't assume the form was posted from your page, anybody can write a page of their own and post to this one).
function okToRegister () {
$register_username = $_POST['register_username'];
if(strlen($register_username) < 4) return 1;
if(strlen($register_username) > 20) return 2;
$register_banned_query = "SELECT * FROM banned_chars_words";
$register_banned_result = mssql_query($register_banned_query);
while($register_banned_row = mssql_fetch_array($register_banned_result)) {
$words = $register_banned_row['words'];
$chars = $register_banned_row['chars'];
if(strstr($words, $register_username_l)) return 3;
if(strstr($chars, $register_username)) return 4;
}
$register_username_l = strtolower($register_username);
$register_check_username_query = "SELECT * FROM users WHERE username_l = '$register_username_l'";
$register_check_username_result = mssql_query($register_check_username_query);
if (mssql_num_rows($register_check_username_result) != 0) return 5
$register_password = $_POST['register_password'];
if(strlen($register_password) < 6) return 6;
if(strlen($register_password) > 20) return 7;
$register_confpassword = $_POST['register_confpassword'];
if($register_password != $register_confpassword) return 8;
$register_fullname = $_POST['register_fullname'];
if($register_fullname == "") return 9;
if($register_fullname == " ") return 10;
$register_email = $_POST['register_email'];
if($register_email == "") return 11;
if(!ereg("^([0-9,a-z,A-Z]+)([.,_]([0-9,a-z,A-Z]+))*[@]([0-9,a-z,A-Z]+)([.,_,-]([0-9,a-z,A-Z]+))*[.]([0-9,a-z,A-Z]){2}([0-9,a-z,A-Z])?$",$register_email)) return 12;
$register_gender = $_POST['register_gender'];
if($register_gender == "") return 13;
$register_country = $_POST['register_country'];
if($register_country == "") return 14;
$register_agreeterms = $_POST['register_agreeterms'];
if($register_agreeterms == "") return 15;
$register_agreepolicy = $_POST['register_agreeprivacy'];
if($register_agreeprivacy == "") return 16;
return 0;
}
if ($register_process == "1")
if (okToRegister() == 0) {
do stuff...
}
else {
....
}