Usually (with a monitor anyway) the display has to be set up to be used with different resolutions. If there isn't an automatic adjustment button on the device itself when you are running in a particular resolution (the default for DarkBASIC Classic is 640x480) then you'll have to uses the screen sizing controls of the display device to fit the view in properly. There should be some kind of menu or setup control set on your monitor (TV) itself.
The steps would be:
1) launch darkbasic
2) use your monitor (TV) screen size adjustment controls to position the screen properly (or the automatic adjustment control)
There may be controls through windows as well if the controls aren't on your deisplay device.
Use the Control panel and choose the Display Option. Under Settings, Change your resolution to 640x480. Use your display card settings to position the screen properly.
When you're done, save your changes. Now switch your resolution back to whatever it was.
Enjoy your day.