you may have to disable the new Windows 8 secure boot feature in your Windows 8 machine's BIOS settings.
To do this go to the Charms Bar in Windows 8 and select Settings > Change PC Settings > General > Advanced Start Up. On the next screen select Troubleshoot > Advanced Options > UEFI Firmware Settings (this option is available only on new Windows 8 machines).
Once you're set to downgrade, just pop in your Windows 7/Vista installation disc or USB drive and install the system as you normally would. When prompted for a product key, use the Windows 7/Vista key that came with the older OS' installation discs and not the newer Windows 8 key; however, you will need your Windows 8 key after Windows 7 is installed.
