|You can't take the "built-in" administrator account out of the "Administrators" group. That's where it belongs and is likely why you got an error message.|
I'd definitely take it out of the "Guests" group, though.
Leave it a member of "Administrators" only, then change or remove (temporarily) the password to get access to the account.
Of course, you shouldn't be using the default Admin account as an everyday user account, so you shouldn't have any files under ownership of that account. The default Admin account is meant for management and maintainence of the operating system and group policies, not common user-level tasks.
You can set up a different account and take ownership of any files currently allotted to the Admin account. Google for "take ownership XP" for instructions on how to do this.
Please let us know if you found someone's advice to be helpful.