Shit happens, bits get scrambled, permissions go awry! When you upgrade the operating system to a new service pack level, new versions of system files overwrite older files of the same name.Also, new data about the upgraded files must be written to the machine-wide portions of the identity does not possess full control over all branches of the Registry, thus, you got an Access Denied message.Right click, one at a time, on each of the five master HKEY names, then choose "Permissions." Find your administrator-level user name in the list of users and make sure that when highlighted it shows "Full control" under , in the "Permissions for " options field.If it shows less than full control - try checking the box labeled "Full control" (under Allow) and click Apply.I thought I was Administrator, but in reality you probably aren't.If your logged-in account identity is labeled "Computer Administrator" (under Users and Passwords - Account Type) it is in fact a member of what is known as the "Administrators Group." That group account has ." The actual true "Administrator" account may possess these required permissions, or at least allow you to override insufficient permissions as needed (See "Taking Ownership" in my extended comments).You may be doing this through Windows Updates, or by downloading the complete SP3 upgrade file from this Microsoft XP Service Pack 3 Download page.At some point during the upgrade process it halts and a message box appears - telling you the following bad news.
Even if you have complete and full Computer Administrator privileges and are logged onto the actual "Administrator" account, you may still have to "take ownership" of the four main branches of your Windows Registry, and/or the Windows Directory and it's files and folders, to successfully install XP SP3.Access Denied errors during a service pack upgrade are mainly caused by insufficient user privileges, either for overwriting system files, or the Windows Registry, or both.What may have happened is that operating system files and folders, and/or branches of the Windows Registry have been assigned to "" account you are using.If you don't have any anti-virus or anti-spyware programs you can use an online scanner from Trend Micro, called House Call.Or, you can download a free 30 day trial of Trend Micro Internet Security (PC-cillin), install and update it, then let it scan for and remove all threats it finds, rebooting as instructed.The first thing you should do before upgrading to XP SP3 is to make sure all of your hardware drivers are up to date and that you have installed all applicable Windows Updates.