disappearing polylines

Using ACAD2000 and for some reason, on one computer, the
polylines revert back to what appeaer to be normal lines -- even
though an inquiry shows that they do have a thickness -- each time I
open the drawing. If I pick one to edit, its thickness reappears until
the next time I open the drawing it's back to no width again. Same
thing with the arrows on leaders. If I modify one, it reappears.
This drawing works fine in other machines with autocad. I'm thinking I
may have accidentally changed a variable. Any ideas?
paul