Had this error while trying to update a Windows 2000 SP4 test box.  As it’s a test box, it has the bare minimum installed.

Microsoft’s solution?

• clean the Internet Explorer cache
• Delete some files in the WindowsUpdate directory
• Delete some DLL’s.

None of those worked.

The actual solution which worked for me? Install Internet Explorer 6.

Other things I could have tried?  Autopatcher, which I wrote about here. Except that Autopatcher no longer supports Windows 2000.  🙁
Or apply the patches one by one, after running an MBSA scan? Maybe.

IE5?  Well I did say it was a test box.