Alan has an old Dell computer and he is having trouble using Windows 10 because of the video problems. Leo says that Windows should be able to automatically read the native resolution and adjust accordingly. For some reason, Windows thinks the aspect ratio is wrong and it's stretching it to widescreen. It could be a driver issue. It may also be the text file that describes the attributes of the monitor. It's called a monitor driver.
Leo recommends reinstalling the video and monitor drivers first, but if that doesn't work, then he'll have to try a full reinstall of Windows 10. Alan can get the Windows 10 ISO from Microsoft at microsoft.com. He can even run the repair utility while installing and see if that will fix it.