You may need to reset the registry and file permissions if you experience Access is denied error when installing a Windows XP Service Pack or update. (Example: Service Pack 3 setup error. Access is denied.)
Hint: If you’re seeing the error Service Pack 3 setup error. Access is denied when installing Windows XP Service Pack 3, to verify if the error is caused by incorrect registry permissions, open the file C:\Windows\Svcpack.log using Notepad and look for the text Access is denied or DoRegistryUpdates failed.
Note: Access denied errors can also be caused by third-party applications, especially anti-virus or anti-spyware applications. So, make sure that you close all the anti-virus and anti-spyware utilities temporarily and then install the Service Pack. This helps in most cases. You should reset the registry and the file permissions only if necessary.Read more