If you get the default grass everywhere it could be in some cases that your graphics card is unable to draw and use masks properly. Could try a graphics card driver update if you fancy such a thing but sometimes it doesn't do much either.
This is not something written in stone exactly, but computers older than from around 2005 tend to bump into these issues. Since you said it's old it could be that the age of the graphics just don't have what is needed to draw the terrain.