You have both the Proportional Grid, the more solid lines, and the grid, lines that look like dots. Turn off the grid using the View/Hide Grid menu or Ctrl/Cmnd + '
Turn off the Proportional Grid by turning off the option found under the Grid and Guide options between the Magnification Ratio and Toggle Mask Visibility icons in the bottom left corner of the Composition panel.
There was a bug in a previous build that fouls up the preview with Guides turned on. Then you should update your current version of After Effects.