This is the best option.

You can install any OS on your PC, considering what you have already paid for your W8 Computer you could create a dual boot system and keep W8 and install W7 as well. It's a good idea to install one of the programs like start8, classic shell or god mode first to give you access to all the control panel items (there are 250 of these) Now click Start> and in the search box type "create and format hard disk partitions" When this opens right click on the partition and select Shrink the volume. After the new partition has been created right click it and select create a new simple volume this will format it and assign a drive letter for it ,it will be called "new volume" you can change the name if you wish but it's not necessary. You can now insert your W7 installation disk in your dvd rom drive and restart your PC. On boot up you will be prompted to "press any key" and then set up will begin, Select "Install now" as the set up loads you will be given the option as to which partition you wish to install it on. Just select the new partition and that's it. Just remember that after each restart the press any key option will appear again, IGNORE this and just let it run until complete. You can install easybcd to set which OS you want to boot up has default.