First of all you need to understand that erasing (wiping) a hard drive might be more than you want to do. Hard disks often include other partitions than the partition you are actually concerned about, but.......
If you are absolutely sure that you want to wipe the drive and perform a custom clean install and you are certain you have the correct media and product key to do so with.....
Boot your computer from the installation media and when you see the pick your language screen, hold the shift key and press F10
At the command prompt type
diskpart
hit enter and give it a second or two
at the diskpart prompt type
list disk
hit enter
observe the disk numbers (if you have more than one then you need to make absolutely, positively sure you know which disk you want)
now type
select disk 0 (assuming there is only one that would be the correct selection, if there is more than one then again, make sure you know what you are doing)
hit enter
at the prompt that says disk 0 is now the selected disk type
clean
hit enter
Goodbye to everything.
Hope this helps.
One more time..... be careful
