As long as you bought a copy of Windows 7 and it's bootable, restart with the 7 disc in the drive, make sure your BIOS is set to boot to CD/DVD drive first and run through the install, deleting and creating the Win8 partition for 7 to go on. Make sure before you do this that you get a list of the hardware in your machine and download all the drivers first (most importantly the chipset, video and wireless drivers, you can always update after the system is running again...