there are a number of ways to delete existing partitions 1. winP

2. Boot from a Windows installation DVD and there will be "Advanced" options to delete existing partitions. If this does not work try method 1: Boot from a WinPE cd and wipe off the harddisk with "Clean all" command in Diskpart. if you don't know how to use Diskpart, Here is the steps:

a. Create a txt file with the name deletepartitions.txt using Notepad, copy and paste the following, Save on a USB thumb drive. 
    select disk 0
    clean all
    exit

b.insert the USB thumb drive to the computer you want to reinstall. Boot from WinPE,
c. Go to the drive of USB, if the drive path for the USB drive is E:, just type E: ENTER
d. RUN diskpart deletepartitions.txt
e. Exit

Restart the computer and boot from your Recovery DVD and you should be able to proceed with  a fresh recovery or installation.