The reason your computers updated to Win 10 is because you have Windows Updates set to
automatic.
I would recommend the "Check for updates but let me choose..." option.
Go to Control Panel > Security > Windows Update > Change Settings.
If you don't do that, the upgrade will happen again on the next cycle.
Apparently this option will
NOT be available in Win 10.
The last batch of security updates I received had "Upgrade to Windows 10" slipped in at the end, which I deselected.
You still have until July 29 to decide, after which the upgrade will no longer be free.
Try it; you might like it.
If you wish to keep Win 7 intact & get Win 10:
*
back up your data to a flash drive
* add two partitions to your hard drive
* load Win 10 into partition 2
* move your data to partition 3
http://www.howtogeek.com/197647/how-to-dual-boot-windows-10-with-windows-7-or-8/
Good luck.