What you can do is press Win+C to open up the side ribbon. Choose the Settings button, then at the bottom click Change PC Settings. When that new window comes up, go to the General section, and at the bottom there is an item called Advanced Startup. Click Restart Now, and there should be an option called "Use a Device" and that should let you boot from a DVD/USB to wipe it and re-install Windows 7.
But if the main reason you do not like Windows 8 is because of the Metro Interface and no Start Button, I recommend buying and installing Start8 by Stardock. It gives you back the start button, and also allows you to bypass the Metro stuff when logging in, so you never have to see it.
Here's the thread I started about Windows 8 and some of the problems/solutions I had
Sager NP9370
256GB Samsung 840 Pro SSD + 750GB 7200RPM Secondary HDD
nVidia GeForce GTX 680M 4GB VRAM
Intel Core i7-3630QM 2.40GHz
16GB RAM
Mouse: MadCatz R.A.T. 7
Keyboard: Razer Lycosa Mirror
Additional Monitors: 24" 1080p Insignia LED/LCD TV and 21" Acer LCD Monitor
Windows 7 Ultimate