Access Denied When Installing SP3 For Windows XPI recently tried to upgrade a Windows XP system to SP3 and received Access Denied during the initial extraction of the files. I then tried to install SP3 for Windows XP from the Windows Update site but that update failed as well. I found this solution to Access Denied when installing SP3 for Windows XP. It mentions this soulution for a RC2 version of SP3 but I fixed the access denied error on my production Windows XP version. First you back up the registry by running regedit, click file and export. Make sure you have "My Computer" the top most part of your registry highlighted. Then click the link to download the SubInACL.exe from Microsoft. Then run the proper script to make the changes and when finished re-run the SP3 install.
Update, this did not work in my case where I got Access Denied during the extraction part of SP3. I have McAfee Virusscan 8.7 installed on my PC. I right clicked the Vshield icon and disabled the on-access scanner. I could then re-run SP3 for Windows XP and it completed installing the service pack.