If however you really want to install windows7 the first thing you should do is check the website if there are drivers for windows7 and the laptop you are looking to get. Chances are if it is a generic consumer laptop there won't in which case you are best of staying with windows8. If it is a business laptop, chances are good they provide drivers for windows 7.
When you boot to your windows 7 disc, you should see a couple of partitions, there will be one for the recovery (how its labeled im not sure) but then there will be the actual OS which you will be able to identify based on it being most of the size the hard drive your laptop shipped with. you could safely delete this partition and install the OS. Then if you later on want to go back to win 8, you can give that partition recovery a whirl. hope this helps