Getting Trigger Happy with SQL Server 2000

I am creating an insert trigger in SQL Server 2000 that works if I insert a new user through the query analyzer but when a new user is created through the web app I get a Foreign Key constraint error. I'm new to triggers so I'm hoping the fix will be straightfoward or maybe something I overlooked.  

Is there a way to create a trigger that fires after an insert is executed so it won't interfear with the constaints?

Here is my table structure:
table: APPUSERS  (tie between a particular app in this case the fifth one and the user id when on that app)

AU_ID, AU_APP_ID, AU_USER_ID,
-----       -------------    -----------
466           5                        635
467           5                     659
856           5                     1117

Here is the trigger:

CREATE trigger trig_INSERT_AppUsers
on dbo.appusers
for  insert
AS

IF update (AU_ID)

BEGIN

             DECLARE @appid as int
             SELECT @appid = (SELECT AU_APP_ID FROM INSERTED)

         IF (@appid = 5)
      BEGIN

            DECLARE @USR_RET_ID as int
            SELECT @USR_RET_ID = (SELECT AU_ID FROM INSERTED)
      
            INSERT INTO appuserprivs
            (
                 AUP_AU_ID,
                 AUP_AP_ID,
                                AUP_Allowed,
                                AUP_CreatedDate,
                                AUP_CreatedBy
            )
            VALUES
            (
               @USR_RET_ID,
                        57,
                                0,
                                GetDate(),
                                0
             )
      END
 END


Here is the error that is being thrown:

INSERT statement conflicted with COLUMN FOREIGN KEY constraint 'FK_AppUserRoles_AppUsers'.
The conflict occurred in database 'OConnordata', table 'AppUsers', column 'AU_ID'.

The problem is in creating new users. If I don't use the trigger, I can create one just fine but using the trigger causes the FK error.

I'm using coldfusion MX 7 on Windows OS and MS SQL Server 2000.

-Eddie