Re: Will Windows 8 boot up if...
That depends on details you have not provided.
Is Windows' Boot Manager on the drive containing Windows, while GRUB is on the drive containing Ubuntu? If so, then simply telling your BIOS to boot from the drive containing windows should bring up Windows, regardless of the state of Ubuntu or GRUB (and regardless of whether the Ubuntu drive is still connected).
If, however, you replaced Windows Boot Manager with GRUB, then no, your Windows will not boot up without some work if you somehow mess up GRUB. My suggestion in this case is that you boot from a LiveDVD or USB and run Boot-Repair.
Jane, stop this crazy thing!