Dual Boot

This is a two part question, and please note I do not have Win10 myself. In fact I do not have direct access to the computer I am talking about although I do have the full technical details of it, including all drivers, etc.

Firstly,
If Win10 is already installed on a machine is it difficult to set up a dual boot with Win7 or WinXP or better yet, Ubuntu 15.04. Could anyone suggest a simple tutorial, preferably video..
I have found lots of articles installing Win10 on Dual boot AFTER installing Win7, but not the other way around

Secondly,
If I was to install WinXP, and various software, on a separate HDD, and then plug that HDD into a Win10 machine, how best could I proceed in setting up a dual boot.
So far I have found that in the Bios options I can change between which HDD to boot first, but are there any other options available?

Could VMware Virtualbox be an option? My past experience with this has only been on laptops and always had problems with graphics or sound. The main reason for wanting a different version of WIndows is Win10 does not support the graphics card/drivers needed for OpenGL

Many thanks for any adive
Alexis

Alex, years ago, I had a shop install my legal copy of XP onto my new Win 7 computer. Later, I added a third partition for my legal copy of Win 8. Later, I upgraded 7 to 10. All three partitions have always worked well. I have had problems with virtual machines in the past and find that partitions are better.

I can't recommend a specific tutorial for 10, but if you type it into a search engine, you can find one suited for your OS and your machine.

Now, you do realize that if you installed 10 on top of 7, you can't use your copy of 7 for anything else.

Many thanks for those comments.

The problem is you are both saying that Windows 10 has been installed 'on top' of an earlier windows version.

I am trying to install Windows 7 AFTER Windows 10 has been installed and from what I can see, this is not possible with Windows 10. Or is it? and if so, how?

Likewise the EasyBCD is used for creating boot menu for O/Ss that have already been installed. or am I misunderstanding it. Can it install O/Ss as well?

Again, thanks for the earlier replies

Beautiful. Thanks very much for that

Alexis, I wonder if you are trying to ask: If I install 10 on top of a legal copy of 7, can I then legally install that same copy of 7 on another partition? The answer is "No."

I still have my legal copy of 7 that I installed 10 on top of. If my computer someday dies, I can then legally install this same copy of 7 on another rig.

Not at all. As I first stated. The computer in question has Windows 10 installed on it and I am trying to get an earlier version of Windows installed as well. But it appears Windows 10 does not let you do that. At least not except as described above.