1. Start Windows 8
2. Hold shift and click Restart
3. Next boot up you see menu , go advance options
4. Select ‘Disable driver signature enforcement‘ from the list
Can test your Win7 drivers now but there will be watermark to show you're in test mode. Hacks are there to remove these watermarks but I don't think it's recommended to leave PC in this state for security reasons.
Hope this helps.