My friend has a lenevo z51-70 which he wants to dual boot with win 7 ...he got 8.1 pre installed in the laptop but he wants win 7 for compatibility with games ...plz help ..(win 7 is in a bootable pendrive )..reply fast and thanks for your time...( He has 4 partitions with main drive where OS is installed is 146gb and other 3 drives are 256gb)