If U mean just the desktop, then right click>properties>desktop >fit to screen
If you mean the screen, then it could be your monitor or your video card.
It may be resolution settings. Right click desktop>properties>setings. Try changing your resolution.
If not, on some PC's, the default scaling of the needs to be adjusted.
you should have a video card panel, (you can usually see your graphic panel in the taskbar, or by right clicking the desktop, or in control panel)
Find it and open it. Look for an option like
Panel Fit, aspect ratio, full screen, scaling options, etc.
(not knowing what graphic card U have, I can't say exact)
If you don't find it, post back the name of your video card
(to find the name, just click Start>run, type dxdiag and press enter. Look at Display, that is where U find the name of your video card)