Windows 8 PCs come with a new security feature called secure boot. You’ll have to disable it in CMOS setup before you can install any other OS. Once that’s done, insert the Windows 7 DVD into the drive and restart your PC. It should ask if you want to boot from the optical drive. Press whatever key will do it.
It might not ask to boot from the disc. Enter the CMOS setup again and make sure the optical drive is setup as the 1st boot device. You will want to download the Windows 7 drivers for your computer before you begin. Save them to a USB flash drive or burn them to disc. After Windows is installed, install the drivers from your disc, then run Windows Update and download everything it suggests.
I don’t recommend installing 7 over 8. 8 is faster to start, has a number of security improvements, and will have support from Microsoft longer than 7. On top of that, you can make Windows 8 work just like Windows 7 which will save you the hassle of actually installing 7.
Install Classic Shell.
http://www.classicshell.net/
It gives Windows 8 a start button and start menu. You’ll practically thing you were running Windows 7 after you disable the hot corners and enable boot to desktop. Oh, and make sure to disable gestures in your touchpad/trackpad settings.
Microsoft will be restoring the start menu for Windows 8 in August so you won’t need hacks like Classic Shell after then.