Did you install it from inside Windows (using mint4win), or did you do a real installation (booted from the Linux Mint installation DVD / USB, and installed from there). If you have did a real installation, just boot again from the Linux Mint installation DVD / USB and start GParted from the menu. You should be able to resize partitions with that.
You may have to right-click the partitions you want to change the size of, and select to unmount them. If it complains about partitions being in use, first run from a terminal the following to disable the Linux Mint installation DVD / USB using your hard disk's swap partition:
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sudo swapoff -a -v