- Check this tutorial.viewtopic.php?f=42&t=126142
- Check this post, has two methods.
If Grub is installed to your MBR then you can use partition manager (Gparted etc) to format the Windows partition and extend the space to Mint.
I hope you have a seperate home partition you can just extend the home partition. Else in case of only \ (root) partition you will have to do on a live DVD.Make sure your data is properly backed up before formatting the disk.