You can just boot Ubuntu, install GNOME Partition Editor (aka gparted) and use that to format the Windows hard drive (to ext4, probably). I would then recommend you to copy everything under /home to the newly empty hard drive and set that as the new location for /home. You can then simply delete the old /home folder (you might want to rename that before you set the new one as /home’s mount point) so free up space for the system. A big benefit is that you can do reinstalls of Ubuntu and re-use all your personal files and settings (though a backup is never a bad idea – in fact, you should set up a regular back-up by default).
Important to keep in mind, though, is that you probably only want to do this if you have some knowledge of partitioning (or have a recent back-up of everything and know how to restore it), because you could wipe all your data if you do something wrong.