Referring to your tags I guess you're running Windows 7.
When you connect your external LCD screen you might use clone mode. Clone is a 1:1 display of the same contents on two screens. Therefore it is limited to the smallest resolution of all displays attached.
In order to use the full resolution of your external screen you can either:
- Use Extended desktop
- Use one screen only
The first option will operate both Screens independently and therefore allow different resolutions and different screen content on each of them.
The second option allows you to use the native resolution but only on one display (the other one will stay off).
To switch between modes in Windows 7 you can simply press Win+P keyboard shortcut (the windows key button and
P). Then select