You need a boot manager to fettle the Windows 7 bootloader. Check out:
I used this software in my marathon of sorting out an SSD (I will post this War and Peace epic when I’ve finished writing it up; a future treat for all forum members). It did crash a few times, but always seemed to make the required change, then crash. Also, I was getting myself into a right mess; doing something “simple” like setting up a dual boot should not cause anywhere near as much grief.
At least you can read about what you need to do from the link, even if you try other software.