How do you want the checkboxes to work? When you check more than one would it return more results or less?
I think the problem was probably with your initial query. I'm guessing it should have been something like:
"SELECT tablename * FROM tablename WHERE tablename.country = ? AND (tablename.company = ? OR tablename.company = ? OR tablename.company = ? OR tablename.company = ?) ORDER BY vrzndw_datasheet.col_company ASC"
otherwise as soon as you leave one checkbox unchecked the query would return no results.