Just restart the system with the 7 DVD in the drive. Pay close attention to the screen for it to say: "press any key to boot from DVD" press any key.
Select to install windows after the GUI loads and you will be presented with a choice of what partition to install to choose the one you want then choose to "format and install"
This will wipe your drive completely and reinstall windows...... Beware all data on the partition you select will be DESTROYED!