|Your display resolution is not set correctly.|
All LCD, LED LCD, and Plasma displays (for a PC monitor input to a TV) have a specified "optimal" or "native" resolution at which the display looks the best when set to - at all other resolutions the display will not look as good, and the text will not be as crisp and clear in Windows.
If you chose the wrong resolution, you can have areas of the display where there is no video displayed.
If you don't know what your display''s "native" or "optimal" resolution is, tell us what the make and model of your external monitor is, or what the make and model of your laptop or netbook or all-in-one desktop computer is.
The specific model of an external monitor is usually on the label on the back of the monitor.
The specific model of a laptop or netbook is usually on a label on the bottom of it, but in the case of a Sony model, you also need to quote other the things on that label.
To fix your display problem....
Remove any bootable CDs or DVDs you have in drives.
Press F8 repeatedly while booting the computer until a black screen with text on it appears.
Choose Safe Mode, or Safe Mode with networking.
(If you don't see that, some computer bioses have another use for pressing F8 while booting. e.g. You may see a screen about choosing which device to boot the computer from. If you see something like that, READ the screen and find what it says about which key you press to exit that screen, e.g. Esc.
Press that key, then immediately press F8 repeatedly until you see the proper screen with Safe Mode etc. on it.)
You will see lines about things that are loading, then the list on the screen will stay the same for a short time.
Then you will see the screen where you Login to a user.
Choose a user.
When the desktop screen has loaded, it will have basic VGA video.
RIGHT click on a blank part of the main desktop screen, choose Personalize, then Display Settings.
You can change the Resolution there.
Click on the blue highlighted line "How do I get the best display" if you have no idea which resolution to choose, for Help info.