If you boot from the dvd and click install now then when you get to the hard drive list section simply delete ALL the partitions. THIS WILL DELETE ALL DATA CURRENTLY ON THE PC. Don't mean to shout, but that part is really important to grasp beforehand. Then create an appropriately sized partition. Windows will also create a first partition about 100 MB or so in size for the system disk containing boot files. It sort of hides this partition to protect it from malware.
Then proceed with the install. One note, be sure to download the motherboard drivers
(VGA, LAN, Wireless, Chipset and so on) BEFORE you do this and save to a usb stick or burn to a cd. Make sure it is readable before you delete partitions. Windows 7 may or may not have all the necessary drivers on hand.