If you were happy with [K]ubuntu as it was before formating, you can just reinstall into the same partition.
But to play safely, backup your Windows partition before installing (to an external drive or at least make a recovery disk unless you have one already). Editing partitions and installing an OS is risky ...
Boot from the install CD, and start installing. At the page about partitions, select 'something else' which lets you select the previous partition (reuse as /, and reformat it).
If you want to change the partitioning, this is a good time to do it.
You can also reformat from a live session (before starting to install). Then use the GUI tool gparted, if available directly, otherwise you can install it temporarily into the live session with
and then it is there to use.
sudo apt-get install gparted
And when you (or he) is happy, make a backup to an external drive.