It appears that Mint is no longer an option for the wubi installer that installs a linux based distro within window, so at this point I would recommend looking at resizing you window partition and installing a physical installation of mint on the same box on a new partition.

There are two options you can try. You can create a new partition on your harddrive, or if you are using the entire harddrive, resize it as mfillpot suggested, and install mint on that partition.

The second option, which I think is easier, is to install mint on a virtual machine using VMware player. This way, you avoid any complications with partitioning or dual booting.