Burn the ISO to DVD or write to flash stick using this tool: Windows 7 USB-DVD Download Tool
Boot the DVD or flash stick by setting it first to boot in BIOS setup, or using the one-time BIOS Boot Menu key given on first screen, e.g Asus is F8.
Choose Custom Install, then Drive Options to delete all partitions, create new ones as you wish, then format before installing Windows 7 to the first partition after the 100mb System Reserved partition which will be created. Illustrated steps here: Clean Install Windows 7
When you boot the DVD/stick it will scan the HD to see the previous OS, allowing use of the Upgrade version key up front during install. Only if the HD is wiped or new does it require leaving the key blank up front and doing the quick registry workaround given here: Clean Install with a Upgrade Windows 7 Version