A the title may suggest, I've already partitioned 10GB of my C drive, and installed Linux. But I really don't know anything about this 'GRUB'.

Where can I install it?
Does it support my ancient motherboard?
Is it easy to install? If not, how?
Is that every hoop I need to jump through lol?

Curse those infernal nonprofit open source software developers!

-Thanks everyone!

It's been a long time since I last tried Ubuntu, but the installer should handle all that with little* to no effort from you . Grub supports just about anything -- unless you have some really obscure, arcane hardware, there shouldn't be a problem.

*And by "little effort" I mean, "Tick a box or click a button, maybe."

If you've ever installed Windows (especially XP or older), it should be an absolute breeze... and if you haven't, it should be very easy.

could you clarify what you mean by "trouble"?

As Sam said, the Ubuntu installer should put GRUB on your computer for you.