A friend of mine has a Packard Bell laptop that came with Windows pre-installed. I installed Ubuntu on it and it works fine, but everytime she tried to boot Windows on it (fortunately rarely) it fails and the computer is unable to boot any OS anymore.
Upon boot, the computer stops on a black screen with a GRUB Rescue prompt.
Is there any way to restore GRUB without re-installing Ubuntu? (preferably an easy way as I cannot do it for her and she is not extremely computer literate)
Also (but less important), is there a way to configure GRUB (or something else) so that Windows won't destroy it on every boot attempt?
Reinstalling Grub2: https://help.ubuntu.com/community/Grub2#Reinstalling%20GRUB2
To reinstall GRUB2 see also e.g. this answer.
To avoid this happen in the future we need to deinstall all Windows rescue/security and more software that may overwrite the MBR of your harddisk.
Depending on the software your friend runs on Windows, and in the case she only rarely needs it you may also want to consider to run Windows in Virtual Box. If it is not for gaming there are invaluable advantages over a dual-boot situation (e.g. run Windows without reboot, run Windows parallel, share data on the fly, share clipboard). However a new installation (and registration) of Windows would be needed.