|Might be a hardware problem with the screen. When you first start it up and it's showing the posting screen, is that also a half screen?|
Or it might be a setting issue--go into display properties either from control panel or right click on a blank part of the screen and choose 'properties'--and lower the screen area. You can also try lowering the refresh frequency. Then click 'apply' and OK. If you get a messed up screen let it sit for 15 seconds or so and it'll revert back to the previous settings. Or reboot to safe mode, open display properties and choose to keep the safe mode settings.
Or it may be a driver issue. Post back the specific model number and we may be able to find the drivers.