Metro interface sucks. Downgrade at your own risk. Most likely drivers will give you hard time.
1. you need to own windows 7 keycode. If it's a brand name computer, create restore DVDs, just in case you need windows 8 restored.
2. download all the drivers from motherboard manufaturer's website, or your computer model from manufacturer's website to an usb drive. Computer manufacturer might not have windows 7 drivers for your specific model.
3. Backup all your personal data.
4. boot computer with windows 7 DVD. Reformat drive in the process.