Windows 10 is free as an upgrade on top of Windows 7, 8, or 8.1. You MUST do an upgrade installation on top of the current legit OS (whichever one it is and by "legit" I mean activated and licensed, if it's some kind of hacked or pirated installation you're on your own there)
at least the first time you install Windows 10 if you're intending to take advantage of it being "free" - that's the catch. It has to be an upgrade the first time.
Once you've done an upgrade installation, after that you're welcome to clean install it as much as you want
on the same exact hardware but again that first installation has to be an upgrade. I hate upgrading operating systems so much I've only done it one time and that was Windows 98SE on top of Windows 98 and it failed miserably so I've never done it again (and I've been installing Windows since before even Windows 1.0 came out).
I did the required upgrade install of Windows 10 build 10240 aka the RTM build about two weeks ago when it was first outed to the Insider Preview program because I understood you have to do the upgrade the first time:
I made a backup image (bit for bit copy) of my current Windows 7 installation, then did the upgrade install of Windows 10 (took roughly 24 mins start to finish), then verified it was functional and showed activated, then I wiped the partition and did a full proper clean install of Windows 10 (took about 14 mins start to finish from a USB stick) - it was not necessary to input a Product Key of any kind. There are two points in the installation process where you'll be asked to put in the Product Key and you're not actually required to do so, you can skip both instances completely (there'll be a link to click to Skip on the first one, the second one has a Skip button). Once it's installed fully and you get online, it'll contact Microsoft, verify the hardware hash which is like a unique fingerprint for that machine it's installed on, and once it verifies the hardware hash against the one created when you did the upgrade installation, it'll be activated at that moment.
Simple process, really, but I know that some folks probably have never installed an OS or upgraded one on their hardware.
As for the updates, Home and Pro users are in the same boat with respect to updates: you can't block or delay them on Home, you can block/delay them on Pro but for no more than 8 months else you'll get locked out of future ones. The only way to have complete control over them being installed is to use the Enterprise edition of Windows 10 but that's a bit insane and overkill for most any consumer (and even most enthusiasts, to be honest). Microsoft has a tool now that allows you more fine control over update downloads and installations but it still doesn't allow you to just say "No, I don't want any updates..." forever on Home or Pro, it just adds some options to Home that don't exist natively without the use of that tool.
I just grabbed the 32/64 ISO (it has both the 32-bit and 64-bit versions on the same ISO disk image) earlier in about 15 mins so I'll have it for future reference. I won't be using Windows 10 as my OS, I can't stand it but I'll need to have the ISO around for customers 'cause I damned well know I'm going to start getting calls in the next few days with people who pooched their machines in whatever manner they do after this whole mess gets rolling today.
tl;dr version - If you want to do a dual boot, that technically is a clean install with the two OSes side by side. To do that, you have to do the upgrade installation first, one time, it's not something you can skip - it has to be done as an upgrade the first time. After that, you can clean install for a dual boot setup if you wish. But that very first install of Windows 10 must be an upgrade, it won't work any other way.
