You don't have to format it with anything, the installation procedure will do that for you. It won't 'mess up' grub it will reinstall it.
As regards Windows you can download a windows recovery cd from lots of places, or you can even make your own, which is better still http://www.howtogeek.com/howto/5409/cre ... windows-7/
, so you don't need the original disk to repair windows (although you do to reinstall it).
Boot from your usb stick choose 'something else' installation option and mark the original mint partition as the the / partition and the original swap as the new swap partition. Elect to format both of them. That is all you need to do. You will lose any data and programs on your old mint partition of course.
Fujitsu Lifebook AH532. Intel i5 processor, 6Gb ram, Intel HD3000 graphics, Intel Audio/wifi. Realtek RTL8111/8168B Ethernet.Lubuntu 13.10,Ubuntu12.10 (Unity), Mint16 (Cinnamon), Manjaro (Xfce).