You need a Windows install CD or DVD (and a valid license to use it.) If your Mini 10 didn't come with one, you'll have to buy one. I see that Newegg has Windows 7 on sale at the moment. It's possible to create a USB drive with a copy of the install CD so you don't have to have a USB CD/DVD drive to install.
When you start the install, select the "Advanced" or "Custom" option, delete all partitions, and install to the unpartitioned space.