we reinstalled Win XP and all is now well,
WHY WHY WHY ?
Here you are, handed the golden opportunity on a plate to re-install a system and start from scratch.
And what do you go and do ? Install an obsolete operating system, that is unsupported, and insecure.
The obsolescence of XP has been well advertised. Both in the run up to D-Day and afterwards. Surely this could not have escaped your attention ? It was covered in mainstream media, and its certainly been discussed here on PPRuNe ad-infinitum.
Stop. Right now. Go back and install Windows 8 (or Windows 7 if you must).
Windows XP machines have no place on this earth, and especially if you have any intention of ever connecting them to the internet.
And please don't start telling me about anti-virus or other security measures. They're all good for nothing if the underlying operating system is insecure. That's a fact.
Please also don't start telling me about businesses or governments you've heard about that are still running XP. Been there, done that on that debate.... chalk and cheese, its not a viable argument, its clutching at straws. There's no reason for private indivduals to be running XP, end of story.
By continuing to run XP, not only are you running an insecure system, but you are also not benefiting from the various enhancements made in newer versions of Windows, especially in the security area where Microsoft have made a lot of effort to introduce new security measures that were simply not present in XP, not even in primitive form.
The fact your son managed to get infected by a crypto virus is a big fat hint that you need all the security you can get.