Need Help With Trigger Sql Server 2005

I am trying to create this trigger

CREATE TRIGGER User_Insert ON aspnet_users AFTER UPDATE AS
IF UPDATE (UserId)
insert into users (aspnet_user_id, username)
select au.userid, au.username
from aspnet_users au
where au.userid != users.aspnet_user_id;

Whenever the aspnet_users table has a new user added I want the Userid and username fields from that table inserted into the
the aspnet_user_id and username fileds of the users table.

when creating the trigger I get the following error..

Msg 4104, Level 16, State 1, Procedure User_Insert, Line 3
The multi-part identifier "users.aspnet_user_id" could not be bound.