Thanks for the reply. However, my concern is that SP3 presumably cannot anticipate what future updates of Windows will do. Specifically, say that SP3 changes a certain file and a future Windows update changes it again. If I use that Windows update and then later install SP3, the SP3 install will presumably roll back the more recent update and, as my PC will have a record of having done the more recent update, it will not correct the roll back (if you see what I mean).
So, my question really is, can this sort of thing happen are am I worrying unnecessarily about installing SP3 now - for example does SP3 actually look at the dates of files that it is changing and ignore them if they show at a date that is in the future to the SP3 modified file?