I have windows 8 and do not like Metro interface. I use a freeware called ClassicShell that bypass initial Metro UI and goes directly to Desktop. It also adds a Start button that can be customized to Windows 7 format.
If you want to install Windows 7, below are the recommendations. DO IT AT YOUR OWN RISK.
Before you do anything else it is important:
1 - create recovery DVDs for your current system. Laptop has an app to do that. About 4 DVDs (single-layer). You need these DVDs if you decide in the future to set the laptop to factory state.
2 - you also need to backup ALL your personal data.
3 - download all the drivers from Lenovo for your laptop model and save them in a USB stick.
4 - you will lose ALL pre-installed apps.
You said you bought the cd key. Is it for 32- or 64-bit windows 7?
Now, the bad news. You need a window 7 DVD that matches 32- or 64- bit cd key. If cd key is for windows 32 bit, you need Windows 7 32bit DVD. If 64bit cd key, you need Windows 7 64bit DVD. Any version of Windows 7 DVD will work. It does need to show on label that is Ultimate or Home Premium. You can borrow from friends, if they have one.
Once you have Win 7 DVD on hand, AND made the recovery DVDs, you need to boot from Win 7 DVD and in install process you need to reformat the drive. Follow through the installation. Once installation is complete, install all the drivers you downloaded in you USB stick. Once completed, check Device Manager if any driver is missing and install it.
Good luck.