Installing Windows on a laptop using an external monitor (main display broken)

Why does the external monitor only work if Windows is installed? That's not normal.

The notebook's video card should detect the presence of the external monitor and display on it automatically. There are two primary modes of operation for this: mirroring and expansion. The former simply mirrors the contents of the internal display onto the external display, which is useful for giving presentations to a large audience, or using your notebook after you've smashed the screen. The second option allows you to connect a second, external monitor to your notebook and use it in addition to the internal display, something that us multi-monitor enthusiasts can't live without.

Generally, the choice between those features is controlled with a special key combination on your notebook. Look for a Fn key and another key (generally one of the F-keys) that's labeled with something monitor-ish. Press those two keys together in order to toggle between the two video display modes. If everything goes well, you can put the machine into "video mirroring" mode, and see everything on the external screen that you would normally see on the internal screen.

(Yes, this answer is quite vague. You didn't give us any details about your notebook--what brand, what model, etc. I suggest checking your manual or the manufacturer's website for more details.)