If you simply format the partition.. you will end up
erasing the grub and wont be able to boot on to windows as well..
Be sure you have a windows bootable disk/pendrive before you format it the ubuntu partition.. after that you will need to boot once with the windows bootable disk, and fix the master boot record.... its a quick enough procedure, you can follow
this or
this.
Regards,
Shrainik Jain
--
Shrainik Jain,
President,
CSA, BITS-Pilani