The two thin lines are there because you are have a trinitron picture tube. The trinitron uses a grid of vertical lines to create the picture you see instead of dots like other monitors. The lines are what hold the grid in place
Trinitron® monitors use aperture grill technology, which is a vertical grating. During normal use, the aperture grill distorts with heat buildup due to electrons impinging on the aperture grill. Therefore, one or more horizontal wires (one for 13.9" viewable image size and smaller monitors, two for larger monitors) are strung across the aperture grill, which stabilizes it as it heats up, in order to prevent image distortion. The line that you see is the shadow cast by the horizontal wire.
Although the shadow can be distracting, it is usually not very noticeable, and Trinitron monitors are noted for their exceptional brightness and image sharpness.
For more information, refer to Dell Knowledge Base Article HO1019511 - "Monitor Troubleshooter: Horizontal or Vertical Lines"