Just install Mint after Windows 7 and choose to install a bootloader/grub in the installation, and it should detect your Windows partition and automatically configure grub for you. But if you later re-format say the linux partition and install some other distro you will generally have to reconfigure grub to point to the new kernel and all that for it to boot the new distro, but you could also just install grub again when you install the new distro and then configure it to also boot Windows if it doesn't detect it.
To configure grub you edit the /boot/grub/menu.lst file, or increasingly /boot/grub/grub.cfg if the distro is using Grub 2. There's usually a commented example for booting Windows in those files. Grub itself is installed to the 1st primary harddrive's so called Master Boot Record (MBR) which is a small boot sector that exists in every harddrive seperately from the actual partitions and all that.
I'm not sure how it works if one has two linux distros installed though. Which distro's grub.cfg should you configure in that case?
Maybe you could have a unified /boot partition between the two distros. I suppose that would work, but I've never actually had two distros installed at once before.
Edit: Oh and if you're doing the partitioning manually, linux needs a seperate partition for swap space. Just so you know. Actually if you have more than 8GB of RAM it shouldn't be an issue running without swap but it can still be good to have one at around 1-2GB or something. And if you want to hibernate I think you need at least as much swap space as you have memory for it to work.