[Logician]

I dont know the structure of your "shoutboxuser" table however doesnt it have an primary key with autoincrement? It's very likely that userid is such a field and if it is, your query must be:

mysql_query("INSERT INTO shoutboxuser (userid, username, password, email, homepage, yim, aim, icq, msn) VALUES (NULL, '$username', '$password', '$email', '$homepage', '$yim', '$icq', '$msn')");

If it does not work for you, insert:

echo mysql_error();

after this line to debug..