This is usually due to a bad connection in the ribbion cable between the montior and the reset of the laptop. This may or may not be fixeable by the user, you need to take it apart and see.
If you go a 3 year warranty with it then Dell will resolve it.
I have a Dell Inspiron 4000 running Windows Me, it is almost 3 years old and I still love it to pieces! For some while I've had some trouble with colour distortions on the screen but as it was only on photos and only occasional I wasn't too bothered. Now, however, in any program, the screen often breaks up into fast flickering lines and blocks, a total mess. If I just tap the back of the screen or move the lid forward a little it rectifies itself. I expect this is something loose - but what? Any ideas?