I am using Outlook XP but I am thinking it might be the same.
-In Outlook, go to Tools | Options
-On the Preferences tab click on the Calendar Options button
-Under the Advanced Options Section click on the Resource Scheduling button
-Within there you will want to check the "Automatically accept meeting
requests and process cancellations", then you will also be able to check
"Automatically decline conflicting meeting requests"
>I have a user who wants to have his meeting requests automatically
> accepted or declined if he is already booked at that time. This also
> has to work when his outlook is not open, is this request possible we
> are using outlook 2003 in and exchange 2003 enivronment.