Are your two OS installations on the same physical drive?
Getting different OS's to "not clash" in multi boot systems is easy if you do it the way I've always set up "dual" or "multi" boot systems.
Everybody knows they work better on seperate physical drives, but what works
best is having the drives with any other operating system sitting in a static free bag in a drawer of your computer bench when one of the other systems is booted.
If operating system "A" and "C" are physically disconnected when OS "B" is up and running "B" cannot terminally screw up either "A" or "C"
If you have multiple OS installations that are always connected but choose during startup which you are going to run there are shared software components that if damaged can bring ALL your OS installs crashing down.
I've BTDT (Fixing someone else's "Train wreck", it is no fun and in my experienced opinion (colored by my anger and frustration) at the past results believe firmly only an idiot would willingly place themselves where having it happen is possible.
"Excellence is never making the same mistake once"
So if I want to switch from my Win7Pro x64 to XP I have to shut down and physically move SATA cables (power and DATA) from one drive to another
(I leave my XP drive screwed into the computer, because it's harder to accidentally drop it)
I don't mind improving things but fixing or replacing stuff that FAILS is never any fun.
Just like Raid0 doubles your chances of a system crash due to a hard drive failure having two or more OS installations sharing a common boot loader
increases your chances of getting nothing other than a BSOD some morning...
Generally when it's most inconvenient