After some horrendous mistakes of the past (experience is truly the best teacher, especially when it's a BAD experience) I've made "Safe, not Sorry" my favorite saying. WELL, at least in the top five.
There's absolutely NO WAY in H*** that I would EVER put two OS's on the same HD.
I know, there's guys who have gotten away with it, but it only takes one mistake to quickly make a believer out of a guy.
Here's how I did it.....
First I downloaded the ISO for win 8.
Then I burned that to a new DVD, using "ImgBurn".
Then, I unplugged the power to my main drive.
Then, I plugged in a spare HD that I had laying around and just put it on the desk next to my tower.
Then, I booted up the PC with the new Win-8 install DVD and proceeded through the install.
It truly was very similar to Win-7, but not exactly. I did have to ask the Installer to format my drive as NTFS. I think it was FAT-32, from an old XP install.
Now, I just plug the power into the drive I want to boot from. Neither OS ever sees the other one, so there can be no Hanky-Panky between them. My main drive remains inside the PC, while the Win 8 HD sits outside.
That looks something like this:
In this pic, there are actually two drives connected outside the main tower. In this picture, I'm cloning one to the other.
Now, when I wanted to try Win-7 on my netbook, I had to rig up an external DVD drive and an external HD, both on USB cables.
I loaded my Ghost backup program from the DVD drive and backed up Win XP to my external Hard drive, then.....
I booted up with my Win 7 DVD and installed Win 7.
Two days later, I decided I really didn't like Win 7 on that little netbook, so again I booted up the Netbook with my Ghost boot disk and did a RESTORE of Win XP from my external HD. No files were lost in the process, but a few electrons were slightly inconvenienced.
So think ahead, make a plan and then work the plan.