Hmm; okay. First off, you will need to purchase a Full Windows 7 (Home Premium) installation disk; you may get a copy off Amazon, as follows:http://www.amazon.co.uk/Microsoft-Windows-Home-Premium-Version/dp/B002DUCMT2/ref=tag_stp_s2_edpp_url
Having gotten the disk, you will simply need to boot up using the installation disk, and follow these steps to install Windows 7 (and remove Windows 8) accordingly:
Step 1 - Place the Windows 7 DVD in your dvd-rom drive and start your PC. Windows 7 will start to boot up.
Step 2 - The next screen allows you to set up your language, time and currency format, keyboard or input method. Choose your required settings and click Next to continue.
Step 3 - The next screen allows you to install or repair Windows 7. Since we are doing a clean install we will click on "Install Now".
Step 4 - Read the license terms and tick 'I accept' . Then click Next to continue.
Step 5 - You will now be presented with two options. Upgrade or Custom (Advanced). Since we are doing a clean install we will select Custom (Advanced).
Step 6 - Make sure to "Format" your C: drive partition first, in order to remove Windows 8, then choose where you would like to install Windows 7. Click Next to continue.
Step 7 - Windows 7 starts the installation process and starts copying all the necessary files to your hard drive. Follow the on-screen directions, and you will be good to go.
Hope it helps. Do let me know if there's anything else I could help you with. Thanks!