I was trying to install Microsoft Windows XP Service Pack 3 on a XP machine.
After running for about 15 minutes I got a nasty error
Service Pack 3 setup error. Access is denied.
I tracked this back to permissions being incorrect on various registry keys.
The permissions can be changed from either a virus or malware program or certain security antivurus program that lock down permissions to try to keep out unwanted changes.
Anyway I found this article from Microsoft that lists some methods of fixing this error. The links is http://support.microsoft.com/kb/949377.
The option that worked for me was Method 1 in the Advanced Troubleshooting section.
Basically I had to download Subinacl.exe from Microsoft install it then create and run the small batch file listed in the above article.
The batch file took a while to run but it reset the registry permissions to the proper settings so that I could then install Service Pack 3 without a problem.
This may not be the only way to fix this problem but "ITFIXED it for Me".
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