http://support.euro.dell.com/dk/da/kb/document.asp?DN=FA1011125&tag=A common cause for marks on the LCD is oil and dirt deposited on the keys of the keyboard that can get pressed against the LCD when carried in a carrying case or when pressure is applied to the lid.
Cleaning
There are many cleaners sold specifically as LCD cleaners. These are perfectly acceptable for cleaning LCDs on Dell notebooks. If you do not wish to purchase one of these products designated specifically as an LCD cleanser, the following is a list of what is and is not acceptable for cleaning your LCD.
The following cleaners are acceptable:
Water
IPA (Isoprophyl Alcohol)
Hexane
Petroleum Benzine
NOTICE: The following cleaners are unacceptable:
Acetone
Ethyl alcohol
Toluene
Ethyl acid
Ammonia
Methyl chloride
These might cause permanent damage to the LCD due to a chemical reaction. Some commercial window cleaners contain ammonia and are generally unacceptable.
Clean the computer’s built-in LCD display with a soft, clean cloth and commercial window cleaner that does not contain wax or abrasives. Apply the cleaner to the cloth, and then stroke the cloth across the display in one direction, moving from the top of the display to the bottom. If the display contains grease or some other contaminant, use a water-dampened cloth instead of a commercial window cleaner.
Maintenance
Avoid subjecting your LCD to extremes of temperature and humidity.
For long term storage, avoid storing in locations that receive direct sunlight or fluorescent light.
Avoid hard physical shocks to the system.