Help Installing Win8.1

I have a legit ISO from Microsoft, and am using Rufus to make a bootable USB disk. I've installed Win7 and Linux Mint using this method before, so I know my computer/motherboard will boot to USB and install the O/S, but Win8 is giving me special problems.

I get the error message (to the effect that) "Windows 8 cannot install to this drive because it is formatted using the GBT style" (or words to that effect).

I've read some stuff and am pretty sure this has to do with EFI, UEFI, USB Legacy support mode and/or one of the 3 different ways you can use Rufus to make a bootable USB drive. Also it might matter if the USB drive is formatted FAT32 or NTFS. I've switched things in BIOS several times, and have changed the way I made the USB drive using Rufus, and each time I either get the "GPT" error message, or the system hangs and will not boot to the USB drive at all.

My motherboard is a ASUSTek Computer INC. Basswood3G (Socket 775) (Quad core CPU) and so the BIOS is old, pre-EFI I suspect, but cannot be certain. I've looked all over the BIOS for relevant switches and found none that sound like what I've read about, meaning it might BE there, but it's called something else. Also it's a fairly simple BIOS and I don't think I've missed anything. There's not much there.

I have 3 hard drives. One "system" drive has just the single partition, my "second" hard drive has several partitions, including an old version of Win7 that I haven't finished cleaning off and formatting, and a working Linux Mint partition. My 3rd HD is 2 Tbyte "Data" partition used for large files and things I want to keep, but do not want clogging up or slowing down my system HD.

I'm trying to avoid having to partition my primary, "C:" System drive. This project to install Win8 is an experiment. I just want to see if I CAN install it, before I decide if I really want to or not. I could overwrite the old Win7 partition, make a new one on the second drive, or make a partition on the data drive. Again note this is an experiment. If it works, then I'll start making serious plans on how to replace my current Win7 installation on my System Disk with Win8.

Also as a secondary subject, I get the sense that my Win7 installation is "old" and therefore insecure and while I am NOT going to install Win10 (for all the usual reasons), I feel like Win8 would be safer and more secure. So if anyone has an opinion on that area, I'd be appreciative. Thanks in advance.