I too am in the process of transition to W10 now that W7 is losing support. I've had it on my PC dual-booted since it was in Beta, but I've never used it, just kept it updated once a month with the regular patches and new releases. Still don't like it, but finally reached the point where it's got to happen, even though I still think W7 is far superior. It has loads of phone type features I neither want nor need and has abandoned loads of useful historic desktop functionality of Vista/7 making it a great cross-platform OS for MS at the expense of its desktop PC users.
The best way of protecting your existing OS and data when installing a new version of Windows is to make full use of the Volume Label, ignored (sometimes at great cost and inconvenience to themselves) by far too many users.
When you format a new device or partition for use by an intended install, don't ignore the Volume Label field of the format command as so many do. Never rely on disk letters to identify a space on disk. Disk letters are "virtual" labels, they only exist in the mind (registry) of the running OS, and as soon as you boot it, the installation medium is another OS in its own right and will have a completely independent (and different) letter map, causing many a user to overwrite his existing OS with the new one ("I definitely installed it in D, but the bl**dy thing overwrote my C disk !")
Volume labels are physical labels writen into the partition table of the MBR on the HDD, and can be seen and identified without confusion by any number of different Windows OSs (or indeed any other foreign OS as well).
If you haven't given a partition a unique label at the time you created it with format, you can do it with a simple right click/rename in Explorer at any time subsequently, just make sure that the intended destination for your W10 is labelled as such before you start the installation.
