You should have an option in your bios of your laptop called secure boot.
You must disable this first.
You next step it to look for the option of legacy usb devices.
Make sure this is enabled.
Next you want to set the usb pend drive ect as the first device in your boot order list of the bios.
Save all the changed settings before exiting your bios.
Now you must make sure that you format the drive, as windows 8 super seeds windows 7 so there for it will not allow you to install it while windows 8 exists on the working boot partition of the hard drive.
Once you have done this you should be able to install windows 7, and at a later date if you choose to have two os Windows 7 and Windows 8 a multi boot menu will be created.