Using Microsoft Outlook 2003 problem

Hello!
Please guide me…
I am using outlook 2003 and exchange 2007 but now I have a problem in outlook as I have a user who can not set up appointments. When he tries to do he gets an error message " free/busy" . I have tried all means possible cleaning the outlook but all in vain. Anyone with any idea how I can sort this out.
Your ideas and opinions high welcomed.
Thank you…
