Then Micro$oft 'updated' it.
The updater re-boots the PC... straight into a crash and re-boot moebius loop.
Result? A complete new install. Luckily, I know the value of data backup so didn't lose anything.
Windows has determined that your PC is operating perfectly.
Microsoft apologises for this error and is now downloading an update which will ensure that such a situation will never arise again
Windows will have to re-boot (a lot) after the update