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.
Resetting the Registry and the File Permissions
Using SECEDIT.EXE (for Windows XP Professional)
In Windows XP Professional, you may use the following secedit command-line to reset the file and registry permissions to defaults.
For more information, see article How to reset security settings back to the defaults.
For Windows XP Home Edition (and Professional Edition), you may use the SubInACL tool to reset the registry and file permissions. Download and then install the Subinacl.exe (~370 KB) from Microsoft. SubInACL is a command-line tool that enables administrators to obtain security information about files, registry keys, etc.
Download reset.zip, unzip and run the reset.cmd file. This script resets the registry and file permissions in your system.
The file reset.cmd contains the following lines:
subinacl /subkeyreg HKEY_LOCAL_MACHINE /grant=administrators=f /grant=system=f
subinacl /subkeyreg HKEY_CURRENT_USER /grant=administrators=f /grant=system=f
subinacl /subkeyreg HKEY_CLASSES_ROOT /grant=administrators=f /grant=system=f
subinacl /subdirectories %SystemDrive%\ /grant=administrators=f /grant=system=f