The majority of new computers that ship with Windows 8 are EFI-based. On such a computer, you need to restore the Windows boot loader to be the primary one and/or delete GRUB from the EFI System Partition (ESP). You can set the Windows boot loader to be the primary one in various ways, but unfortunately, they all take a lot of explaining and/or are tricky to do. Deleting GRUB from the ESP is relatively easy to explain, so I'll do that:
- Boot a Linux live CD.
- Identify your ESP by typing sudo parted /dev/sda print and locating the partition with the "boot flag" set.
- Mount the ESP by typing sudo mount /dev/sda1 /mnt (substituting the correct device identifier for /dev/sda).
- Type sudo rm -r /mnt/EFI/ubuntu to remove Ubuntu's version of GRUB. If you've manually installed GRUB somewhere else, remove it instead of or in addition to Ubuntu's GRUB.
- Reboot.
When you reboot with GRUB absent, the computer will bypass that no-longer-functional entry and use the next one in its list, which should be the Windows boot loader.
A caveat: If you've used Boot Repair, you may need to re-run it and use its advanced options to undo its renaming operations. If you fail to do this, you'll end up still running your non-functional GRUB.