After installing a hotfix and running sfc / scannow, you may replace some files with older versions because SFC only knows how to retrieve files from the installation media and from Service Packs.

Starting with SP4, hotfixes will register themselves so that SFC will recognize them as valid file sources.

Prior to installing SP4, if you run a SFC command, you must reapply the hotfixes or you may end up with mixed binaries and unpredictable results.