GPMC Error

We have a strange error on our Win 2k3 server. The built in administrator account cannot edit group policy. The other admin level accounts can, but when the built in admin opens GPMC and right clicks a policy and chooses edit the following error pops up:

Group Policy Error
Failed to open the Group Policy Object. You may not have appropriate rights.
Details: Logon Failure: unknown user name or bad password.

This only happens when logged in as the built in administrator (logged on locally at the machine too).
This problem has apparently existed for years. Can someone help?