These issues relate to anti-virus programs possibly interfering with the installation of Service Pack 3; click here for a Microsoft Knowledge Base regarding this issue. I did not sit by the computer but did other things and looked in every 5 minutes or so. Still have a big problem playing games but I have identified that as being caused by outdated Apple drivers - still to be sorted. A hotfix KB for this specific issue is available for download from both this article and the Microsoft Knowledge Base. I thought that I had already seen a part of the installation go through before when I looked in. There was an option to Quit which I took and then restarted the computer and lo and behold XP SP3 was already installed.