From what I've read online, Toshiba put everything on a partition and you access it by pressing a shortcut when the machine loads up.
That’s right but you will have access to this partition with “factory settings” only. Problem is that after own OS version you will not be able to start HDD recovery installation anymore.
I've got a valid machine with a valid serial etc. I just want the old OS back ASAP.
Correct, but this valid serial belongs to OS that you got with your notebook. In Toshiba user’s manuals is clearly described how notebook owner should create recovery DVD and have back-up of original recovery image. Using this disc you can install recovery image anytime you want.