I've just gotten my pc back from a repair company, and when attempting to start it it goes to a black screen with a blinking underscore. It has a new western digital harddrive in place, and I can get it to boot to desktop-but once it restarts, it goes straight back to the black screen. Is it possible to fix this?? Sometimes when I get the boot screen for windows 7 with the logo, then it just sits like that.