I think if I wasn't dual-booting on my netbook simply installing the service pack might have gone OK. Unlike my PC multi-boot which went OK probably because it's Vista/Win 7, the 1st OS is XP & I seem to remember a similar problem installing a Vista service pack years ago because it couldn't find the correct boot files (Vista and later OS's use different boot files).
Now this is weird, I just started it up again from within Win 7 thinking it would start from point "A" again and this time it's going ahead. Maybe it did the repairs, whatever they were when I rebooted last.
It's now happily "Upgrading Windows"...that makes me breathe a sigh of relief as I know that at least my files and settings should be preserved, if not the tweaks. The latter can be done again.
If it stops my XP from booting there will be much gnashing of teeth, but let's not panic unnecessarily.