Cannot load any Windows Updates: KB842773 Error

Hi,

I am running a Windows 2000 Advanced Server w/ Terminal Services. This
machine has not been updated in quite a while, so in order to download the
latest Windows Updates, the latest version of the Update for Background
Intelligent Transfer Service (BITS) 2.0 and WinHTTP 5.1 Windows 2000
(KB842773) needs to be installed. However, I receive the following error
everytime I try and run the update, just when it gets to the point where it
wants to update the registry:

KB842773 Setup Error.
Access is Denied.

I have already tried to do the following, with no success:

I have run the update using Windows Update (through Add/Remove
Programs), and also have downloaded the update and run it manually.
I have run the update logging into the computer as the domain admin,
and also as the local admin.
I added the local admin user and domain admin user accounts to the
Backup Operators group.
I also went to the Local Security Settings/Local Policy/User Rights
Assignment/Backup Files and Directories and added the local and domain admin
accounts, as well as the Backup Operators Group for good measure.

I can include the log file on any subsequent posts if necessary.

Any help or other suggestions would be much appreciated.
Thank you.

Gary