That means boot from the DVD and select Custom clean install. Use Advanced driver
tools to format the drive and install.
If you want to go back to 32 bit anyway, then contact your Windows 7 retailed and request the 32 bit DVD as you are entitled to use either 32 bit or 64 bit with your license. You can also borrow a copy of 32 bit which should activate upon install if it is the same version.