Its asks out of the blue or when you press empty?
Its possible windows has become a little confused in its tracking of deleted items.
When its having one of its empty moments setup the bin as showen in the attached shot.
Apply the changes and then set it back to how it was. You can then rest assured the bin is actually empty.
If it asks after that you could always press yes bearing in mind it wont delete the windows dir because of system file protection. However not all files in the directory are included in the protection scope.
Its a strange one just thought i would give u my rougth thoughts on this
