I have a small scratch on my screen, at the plastic cover level. Is it possible to have the plastic cover replaced without replacing the entire screen?
I would think so. The LCD itself will have a glass cover, and the plastic over-cover would be there to protect the glass. Where did you buy the laptop? You should be able to give their service department a call for a quote.
A company called ScreenTek (http://www.screentekinc.com) can do this for you. The only thing is, they have to have your screen in stock to complete the repolarization of the screen. This is due to the fact that the repolarization process can be difficult and if the first try doesn't work out, they have to have a back up screen for you.
What Kevin said as well. If the screen is still visibile, but it's darker than hell, it's the backlight. Depending on the manufacturer, it may require the entire LCD screen section to be replaced.