I have an old laptop which is out of order. Can I use its LCD screen as a LCD monitor for my desktop PC ?
This would only work if your computer has a connector that can be used as an input for video signals and is still working.
Laptops have no VGA or DVI connection internally, they have usually have LVDS signals that go to the LCD screen. I would be very surprised if that is different on your laptop. So it does not help to open the laptop. Also, your VGA connector on the laptop usually is an output only, not an input, so it will not help to connect it to the computer since that connects two outputs. Sorry for the bad news.
Open it and see what you find.
But beware, they probably don't have any "normal" connectors in there so don't forget your soldering iron.