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