Found this on reddit.

This way, you get your free win10, without having to actually lose your win7/8/8.1 etc.

Handy for those that need earlier systems to run things that can`t on windoze10 or for lack of trusting the newer OS.
Also means you don`t have to pay once is does stop being free.
Good for when you upgrade hardware (gfx cards etc) & need the newer OS in the future.

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1) Boot from windows 10 usb installation stick (on pc with 7 or 8)

2) select language etc. and get to install now screen

3) Press shift+f10 to get to command prompt

4) type following commands

c:

cd\

md vhd

diskpart

create vdisk file=c:\vhd\win10.vhd maximum=32000 type=fixed

select vdisk file=c:\vhd\win10.vhd

attach vdisk

exit

exit

3) then click install now, select custom install and install to the newly created virtual drive and use 7/8 key to activate.

This will create a dual boot menu to old OS and new vhd.

To remove windows 10, run msconfig, select boot tab and delete windows 10 boot entry, then delete vhd.

Thanks for this tutorial, it was just what I was looking for (a way to take advantage of the free upgrade to Windows 10 without needing to replace Windows 7 at this point)

Used a DVD for the install rather than a thumb stick (I have a million blank DVDs and no thumb sticks) and when the installer asked for my key I used the one that came with my current "OEM" copy of Windows 7. Had to create the vhd file on my 2TB external USB HDD since none of my other disks had enough space free but Windows 10 installed just fine on it (even though it gave me a warning saying "Windows cant be installed to this disk"). Booted up just fine and told me it was activated.

So now I can install Windows 10 any time I want.

I used DVD too.
Got plenty around, got a few USB sticks too, but could I hell as like find any of them lol.

Glad it worked for you too :)