Looks like there can be a few issues with upgrading to Windows XP SP3 with some users getting the following message: “Access Denied”.
The cause is certain security software which runs in the background and locks Registry keys as part of the security process. This prevents the update from completing successfully and Windows XP SP3 won’t install. It will give the helpful message that “Service Pack installation did not complete” followed by “Windows XP SP3 only partially installed. Your computer may not work properly” (helpful, huh?).
The solution is to disable the security software and modify the registry so it can be overwritten. Microsoft has a step-by-step instruction set that lays out how to do this – available here Article ID : 949377. It’s not for the faint of heart mind you and if fiddling with the guts of your computer’s innermost workings gives you nightmares and brings you out in a cold sweat you may wish to seek assistance.
More solutions to other Windows XP SP3 issues such as “Stop 0x0000007E” error messages available here.