Evo sta pise na MS stranici za taj update:
The original release of Update 938371 updates several internal components that Windows Vista requires in order to install or to remove Windows Vista Service Pack 1 more reliably. This update must be applied separately before you install Windows Vista SP1 to make sure that Windows Vista SP1 can be installed or removed from the computer. Update 938371 is necessary to install and to remove Windows Vista SP1 on all versions of Windows Vista.