If you have the latest version of Liberty this bug has been fixed. So upgrade. But also the manual fix is easy, edit the liberty.ini file which is a simple text file and put sensible values in for the screen positions. use %appdata% navigate to your Liberty BASIC folder then find the .ini its called lbasic451.ini for the latest version. The screen xy wh appear as a line of numbers at the foot of the file.
If you download and reinstall the latest version, v4.5.1 the ini will be replaced and the problem will go away.