The display driver may be set at the minimum screen resolution.
Clear your screen by minimizing all open windows or just close everything.
Right click the Desktop to bring up Display Properties and adjust the Settings Tab.
Go for the largest screen resolution your display driver supports which should be enough to fill your screen.
If it doesn't, or if there are no higher resolution screens available, there may be issues with your video driver.
You may find there are issues arising because you changed one of the four major components - Hard Drive, Random Access Memory, Video Card or Motherboard (!).