Good catch. That is probably exactly what happened. Windows will always take the entire drive if it can.
Stan caught the essential fact that I missed. You installed Windows 7 after installing Mint. The only way to do this and preserve your Mint install is to provide a partition formatted to NTFS for Windows to install to. Otherwise Windows takes the entire disk.
With Windows installed on the entire disk, it becomes much easter to set up the dual boot with Mint. From Windows run Disk Manager and use it to shrink the Windows partition. You should be able to free up 1/3 to 1/2 of the disk. Before shrinking the partition, defragment Windows to compact it to a smaller area of the disk.
Now run the Mint installer and choose whichever partitioning mode you are most comfortable with.(I always recommend manual partitioning since you have complete control over how Mint is installed but this is sometimes daunting to first time installers). The simplest method is "Use largest contiguous free space". This will install Mint using the entire space you freed up with Windows Disk Manager.
At the end of the installation the Grub bootloader is installed. Grub will detect your Windows installation and automatically set up a dual boot menu. Mint will be the default OS. You will have 10 seconds to hit any key to stop the timer and choose an OS when you boot. If you do nothing during those 10 seconds, Mint will boot automatically.
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