unksol :
put the disk in the drive, select boot from DVD at startup, and when it asks delete the windows 8 partition. Then continue with the install
I tried all that and then spent 3 hours on the phone to Microsoft over numerous days as wel as HP, and neither of them could do it either from the disk or downloading a special one from their website. Microsoft said they can't find an answer as to why Windows 7 won't install and believe me we went through every senario. They said Windows 8 cannot be uninstalled. It was the driver that my MYOB accounting package that would not install therefore I couldn't work my thermal receipt printer in my retail shop and MYOB haven't created the updated version as it is an older printer and as it works I was not going to buy another one.
PS You don't get the option with Windows 8 to format disk or delete windows 8 partition on preinstalled HP computers.
I ended up returning the computer and searching for one with Windows 7 on and now is all good but allot of wasted time.
I don't think that Windows 8 is suitable for retai shops at all it is really for touch screen operators and even takes longer to shutdown having to go to top right corner then settings then shutdown - crazy!