I received this error when I was installing XP SP3 on a machine that was giving me trouble. This particular machine had been updated with SP3 and KB946648 via WSUS previously and even though it had SP3 and that KB installed already, WSUS was saying that there was an error with both of those updates. I tried downloading and installing KB946648 by itself and it didn’t work. The error on that update was “required installation branch was not found in INF…”
I decided to use System Restore on the system to turn it back to before SP3 and the KB were installed so I could apply them manually and see what would happen. Tried installing SP3 first and it caused the error listed in the title.
The solution was to goto the SPOOL folder under Windows\system32 and click on the Advanced security and reapply the inheirited security. For some reason it was not allowing my administrator account to copy the unires.dll file to a subdirectory under spool.
I should note that this machine has a primary disk lettered F:\ even though that should have no bearing on anything…I have noticed that certain programs will freak out if they do not have a C drive to attach to because they ASSUME that every computer has one. Interesting no?