First thing to try: Right click on the desktop, select Properties, go to the Settings tab, and make sure that the resolution matches your monitor and was not set to something smaller like 800x600. If'n that doesn't work, ask again here.
Ok my resolution now is 1024x768. I,m actually using my hd tv for a monitor right now so I'm not actually using a computer monitor. My old computer has xp and I used this tv and the resolution was better. Plus I have dual operating system with linux. And the resolution for linux is way better.