Digerati is correct in that the UEFI bios is there and cannot actually be turned off. You can bypass it by installing the system in Legacy configuration, which is what you did.
Which configuration is used to install depends on how you boot the install Media. It should have options in the Boot Device Menu for either Legacy or UEFI versions of the boot media. You can turn off the ability of the system to see the UEFI options (or Legacy) during boot, but if you do not use the Boot Device Menu, the system will probably boot to the Legacy version by default.
If I may, could I ask exactly what settting you changed in the Bios? I have a P8Z77 board which is similar.
But the UEFI on/off question should not be a factor. If you are getting Blue Screens, you might check the BSOD forum for how to get help using the dump files.