If when you try to install Windows Vista Service Pack2, you get an error message : Service Pack installation cannot continue. One or more system components that the service pack requires are missing, then this article may help you.
Service Pack installation cannot continue
This problem occurs because system components that are required to install Windows Vista SP2 are not present on your computer. However, a common reason for this problem is that the vLite software was used to customize the Windows Vista installation and some required system components were removed.
To resolve this problem, use genuine Microsoft software media to reinstall Windows Vista with Service Pack 1 and then install Windows Vista SP2.
You can get more details at KB968279.
Hope this helps!