Regain denied access

I have a machine equiped with WinXP SP2 which belongs to a domain. I have a user with domain admin rights. I`ve took ownership of system partition and documents partition but denyied access to another user with same domain admin rights to same partitions. My built-in Administrator is renamed and granted full access to the machine. Of course i`ve removed the Everyone group and other users from implicit rights list. Still, I cannot login to the machine,...it accepts the username and password,it shows the desktop,...but logs off imediatelly either I login with domain rights or local administrator. Where was I wrong?!