Generally, this error refers to a corrupted index file or database. As far as I am aware, dBase doesn't care to much about your monitor except whether it's color or monochrome. If you can open other DOS windows then you should look at running just the dbase.exe program with no files open. If you can do that, then it's a corrupted index file. Open each file that has an associated index (use filename) and reindex each file.
There's always a better way. The fun is trying to find it!