My sister downloaded some free Win 10 offer from MS.

This apparently "upgraded" her laptop to Windows 10. She never solicited the upgrade, she was just asked if she wanted it and clicked "yes".

It also didnt reformat her laptop, all her own original files and stuff is still there, I would have thought it unwise to install over a previous version of windows and retain your previous files and folders.

Is this genuine do you think?

Any help appreciated.

Any computer running Windows 7, 8, or 8.1 is eligible for a free upgrade to Windows 10 via Windows Update (an update added a Windows logo near the clock, and if you click on that you can choose to accept the upgrade), provided you do the upgrade before (some time in) July 2016, even pirated copies will get the update and can upgrade. The legal status of Windows 10 is determined by the version you upgraded from - if your Windows 7 is pirated, the Windows 10 will also count as pirated.

Yes it is genuine. MS gave away 10 for free to all valid Win 7 and 8 machines.

It is. Microsoft really, really wants people to use Windows 10, so everyone with Win 7 or 8 gets an offer to upgrade. It's a pretty annoying little notification in the taskbar that can't be turned off and will pester you periodically to get the new OS.
It's a timed offer, though. It'll only be valid for a year (the offer, not the system). If you didn't upgrade by that point, you'd have to buy a copy of Windows 10 if you want it.

It can be turned off though, by uninstalling a specific update and telling that to never come back.

The notification can be shutdown easily - just deinstall this windows update > KB3035583 and click on hide update after its deinstall. That's it.

Thanks everybody for the replies.

I gave the solution to the first reply and +1''s all around for everyone else, very much appreciate the speedy help :)

See, here comes a guy I've never heard of and gives information I was too lazy to bother digging up. A thumbs-up for him.

Good to know. Thank you both.

no, currently there is a "way" in which one can upgrade a pirated W7 to a genuine activated licensed W10

Also, in the first month after upgrading you can roll back to Win7/8.

provided windows does not delete the rollback files to free up space this is true.

Thanks for the extra replies, handy stuff to know, thank you :)

I don't think it automatically does that, you'll have to do it yourself.

I don't recall which action removed them (maybe it was running Disk Cleanup => Clean up system files), but even then it still confirmed whether I really want to remove them, and that accepting it will mean the option to downgrade will be gone.

(I think clean install to the earlier Windows version is still always possible, just that the one-button downgrade is not.)