In XP-Pro unable to install Service Pack 3, getting Access Denied msg !

I have XP-Pro and very recently tried to download and install Service Pack 3 as part of Windows Automatic updates. But after the installation started and went for a while , I got the error msg, “Access Denied” and some concluding msg like, “Now your system will revert back to the original condition as before the update”. FYI I am using my regular login profile which has administrator privilege. Please help as the SP3 is needed to be installed in my laptop for security reasons I suppose. Thanks.