What exactly is the issue? What does the boot error have to do with the black screen / white cursor? Do you mean it goes to a black screen with a blinking cursor in the top left? That's a disk issue, either the drive is bad, or the setup lost it's boot partition info. Download "testdisk" and run it, if it's an issue with the boot partition it may be able to fix it. The computer BIOS may also have a disk check utility, run it and see what it tells you.