Once a partition is marked active, it will be searched for a boot loader by the BIOS. What you have to do now is boot from the Live USB, run Partition Editor and mark as active another partition that already has a boot loader (very likely /dev/hda). Then you configure whatever boot loader that you want to use. Judging by the variety of operating systems that you've got, I presume you're planning to do something like this
, right? If my guess is right, you don't have to use GRUB, so no need to reinstall Mint.