Open up the laptop and unplug the hard drive. Then you can try changing BIOS settings and doing whatever else you need to do. I would also recommend booting from a Mint Live CD or Live USB to repair GRUB. If you boot from the windows 8 DVD and repair everything that way, you will lose GRUB and Mint.
The process I would use is:
Boot laptop using Live CD/USB
Wait until booted
Plug in HDD
Put laptop back together
Boot and check that everything works.
You could also try booting into Mint using the GRUB console and skip the above steps. You'll have to search the internet for a tutorial on that.
Running on a 4-slot toaster @ 60Hz