Some users are reporting that they are receiving an "Access Denied" message when trying to install Windows XP Service Pack 3. I am going to show you today some potential fixes for this error.
One potential fix is to reboot the computer, and then trying to install service pack 3 manually, by downloading and running the service pack 3 install file from Microsoft.
Microsoft XP Service Pack 3
Another fix is assuming you have been trying to install Service Pack 3 with an administrator account. You can try resetting the security settings for Windows XP. A system file may be locked for some reason. You can follow the short tutorial from Microsoft, to reset your security settings.
Windows XP Security Settings
Another possibility would be to reboot your computer then disabling any anti virus, or adware, spyware application(s) you have installed and then attempting to install service pack 3. You can follow the tutorial for this from Microsoft. Go ahead and skip to "Method 2" under "Resolution" since you have already tried method 1 under Resolution.
Microsoft Support XP
A fourth possibility, if none of the methods above work, is to reset the registry and file permissions. This is an advanced method and can be completed by following "Method 1" under "Advanced Troubleshooting" section.
Microsoft Support XP
You can also read through the prerequisites before installing Service Pack 3 if you would like from Microsoft. There are also some additional troubleshooting steps in that article for "access denied" errors when attempting to install Service Pack 3.
Windows XP Tech Support
You might want to keep any of the Windows XP Service Pack 2 updates under add and remove programs in case you want to roll back to Service Pack 2 in the future.
Sometimes KB911895 fails to install after upgrading to Windows XP Service Pack 3. You can browse to C:Windowssystem32drivers and then delete or rename the wdf01000.sys file. After deleting or renaming the file, you go to windows update and select "Custom", then select hardware updates and "HID Non-User Input Data Filter" should be there again. Now select download and install. Hopefully, KB911895 will install successfully.
Another option if that doesn't fix the KB911895 problem is to then boot into safe mode and download and install the update manually from Microsoft.