Here's what my motherboard shows: http://i.imgur.com/jx7kjR4.jpg

Before, it was set at the default which is "EFI Compatible ROM". I changed it to "Legacy ROM" but the installer still loaded GRUB, so I followed jtsn's suggestion and went into the DVD BBS Priorities and set that to legacy also. This did the trick.

Note that the defaults for both were EFI, and when I originally installed Windows it didn't mention anything about GPT or EFI. Maybe it's the way the ISO is made or the fact that I have an MBR drive but Slackware's installer is the only one I've had to do this for. I've had this setup and hardware since 13.0

If the EFI firmware was set to EFI Compatible ROM when Windows was installed then I have to ask, when you repartitioned the hard drive to add Slackware did you use gdisk/cgdisk or fdisk/cfdisk to format the free space you allocated?

The PCI option ROM priority has nothing to do with the boot mode. It allows you to have PCI cards with 64 bit EFI drivers, but such cards don't exist yet, so the option currently does nothing.

While the mainboard seems to have a fancy Windows 7 look-a-like setup program, it seems to lack proper documentation of the options in there. A good printed manual describes the installation of various operating systems in legacy and native mode, how to configure the firmware boot menu, how to enable/disable Secure Boot, how to add you own keys and so on...

That is a rather complex topic, specific to a mainboard model, so good documentation is key.