I successfully got rid of Windows 8, and installed Ubuntu 15.10, but it won't boot unless I use the a usb boot. How can I get it to boot without using the usb?
I might be stating the obvious here; did you actually install Ubuntu or just "try" it by running it from the USB?
Well, if you can have an terminal acess via USB. Try this:
$ sudo fdisk -l
then, see what partition you have the ubuntu installed(ext4 filetype) then:
$ sudo mount -t ext4 /dev/sda6 /mnt (admitting that sda6 is my ubuntu partition)
$ sudo grub-install --root-directory=/mnt /dev/sda
this will reinstall grub and see all the others OS' that you have and organize all of them.