Can't delete a user from SBS 2011

I can't delete a domain user from SBS 2011 from Console or from Active Directory Users and Computers.

Everytime it tells me 'Access denied'(from console) or 'You do not have sufficient privilege to delete..' (from AD users and computers).

i'm using administrator account.

When I try to enable outlook web access for this account, it shows 'cannot update user access to outlook web access.' Hope this can give you a hint.