What is the difference between covalent and molecular solids?
From the naming of the two, you can see that covalent solids do not consist of molecules.
Molecules are moieties that are made up of covalently bound atoms, but the molecules themselves are bonded to each other with much weaker Vanderwaals forces. In covalent solids all atoms are bonded to all other atoms through covalent bonding.
Due to their nature, covalent solids can't simply "melt": that would have to break covalent bonds. This will only happen at very high temperatures.