I would probably try and get another Windows 7 disk. Up until recently you could just download the .isos straight from Microsoft (effectively). You still needed a key and to activate it though.

BUT before you go down that path....

You mention changing the BIOS and it still boots Windows 8.1? So that's the main PC's BIOS? The Virtual machine has its own separate BIOS access that runs after powering on the VM. The VM really is a complete PC inside a program so has all the bits a non-virtual machine has - just most are done in software. Completely separate to the main BIOS.

Virtual Box has the boot order set in Settings->System->Motherboard. Move the CD/DVD-ROM to the top. Leave it and the hard disk checked. It should be that way already though.

Like this:
http://en.kioskea.net/faq/40841-virtualbox-how-to-change-the-boot-order-of-a-guest-os

Also I would remove the extra CD/DVD-ROM you have installed to prevent confusion. Not the G one.

This should be done with VM not running (off)

You shouldn't be rebooting the whole machine each time. Just reboot the VM. Your screen shot indicates you've got as far as booting the machine but you talk about booting 8.1 which doesn't make sense. If you just mean to try out the disc, some Windows 8 machines won't boot from the CD very easily I think due to some Windows 8 secure boot stuff (UEFI).

If still no go, I'd be looking at a new install disc or downloading a genuine ISO. Can you download ~3GB?