Since the questions has come up already more than once,
Following are some links, in no particular order to tutorials/instructions involved in the process.
- How to Uninstall Windows 8, Install Windows 7 on Your PC
- It’s Not Easy, But You Can Uninstall Windows 8 And Revert To Your Old OS
- How to uninstall Windows 8
For those encountering issues with hard drives set with GPT,
You can see if the software MiniTool assists,