The safest and simplest way:
Let's say you want Windows 7 and Windows 8 on same computer,but you are afraid that you do not destroy Windows 8.Do the following:
Add one more HDD to your computer,but disconnect the one which contains Windows 8.Turn on the computer and install Windows 7 on that newly added HDD.Once that has been done,turn off the computer and reconnect the previously disconnected HDD.
When you want to use Windows 7:
When you turn on the computer,go to BIOS and put the HDD (on which Windows 7 is installed) as the first device to boot from.
When you want to use Windows 8:
When you turn on the computer,go to BIOS and put the HDD (on which Windows 8 is installed) as the first device to boot from.
Going to BIOS and putting some HDD as the first device to boot from should not really be a problem since it takes just few seconds once you get use to it.Besides you will not use different Windows OS every time you turn on the computer so you will not need to go to BIOS every time UNLESS you decide to use Windows OS from the HDD which is not the first device to boot from in the BOOT PRIORITY/SEQUENCE.
Fixing problems:
If let's say you are fixing ANY kind of problems on let's say Windows 7,be sure to disconnect the Windows 8 HDD so you are 100% sure you do not mess something up no matter what you do.
Same applies for Windows 8 or vice versa.
Cheers!