When the logon box appears type the user name: Administrator and then type in any password you have assigned to that account.
If you have not created a password for Administrator, leave the password field blank and press Enter, to logon.
An Administrator account should have complete control over all portions of the Registry, in order to upgrade the OS and major components.
Something happened that removed that level of permission from a branch of the Registry required to be writable in order to upgrade the service pack level.
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.
When the Registry Editor opens there should be five items with signs listed in the left pane, each beginning with "HKEY," each representing a different branch of the Registry.However, some programs won't install from Safe Mode and your networking may not be fully functional (Safe Mode With Networking).Therefore, I recommend using the second method of accessing the true Administrator account - the Classic Windows Logon Screen, which will allow normal access to the Internet, with full Administrator privilges and the ability to install SP3 (and run followup Windows Updates).While you can easily access it by rebooting into Safe Mode (tap F8 during boot cycle), you can also access it via the Classic Windows Logon (box, or screen). To temporarily use the classic logon screen, press CTRL ALT DEL on the Welcome logon screen.This will hide the Welcome Screen and reveal the Classic Windows Logon box.Although this article was written during the Windows XP era, the information in it may still apply to some newer versions of Windows, especially the parts pertaining to file, folder and Registry permissions.