First and foremost: If you’re a Windows 7 or 8 user, Microsoft will not email you to upgrade to Windows 10. If you do get an email along those lines, it’s probably a scam and if you download what’s inside, bad guys could hold your computer’s files for ransom.
Cisco Security’s Talos researchers discovered a new scam email, involving what’s known as ransomware, going around that offers to upgrade computer users to Windows 10 for free. While Microsoft is upgrading Windows users for free, the company is only sending notifications to users via their desktops, and is not emailing anyone. If you see an email along those lines from someone purporting to be Microsoft, delete it immediately.
Those that do download the “upgrade” from the email will instead find their computers taken over by ne’er-do-wells who will demand to be paid, likely with Bitcoin, as it’s untraceable.