Can't install software

Hello, I have a Gateway Laptop. It currently has Windows 8.1 pro. I had to replace the hard drive and purchased a disc with Windows 8 in three versions. I didn't know which one came with the lap top so I installed the pro version. After it was installed it asked me for a product key. I don't have a product key for this version. I obtained the product key that is in the computer bios but that key didn't work. I have now obtained set of recovery disks that were installed on the laptop when I bought it. When I try to install the software it tells me that the software can only be loaded in the UEFI mode. I changed the setting and restarted. Then it gave me two choices to pick from. Upon choosing one it then tells me the it can only be done if the setting is in the Legacy mode. So I changed the setting again but then got the original note that it had to be in the UEFI mode. It just goes in circles and I am not able to restore to the original software version so I can use the product key that I do have. Any suggestions?