I thought it was all pretty explicitly described in the original post.... I'll try again:
I had a desktop link.
I assigned hot key combo: <ALT><SHIFT>S to it successfully.
Later that link [file] was permanently lost.
Now I have another desktop link I want to assign to <ALT><SHIFT>S
But I cannot, because something in Win 7 still thinks that combo is still reserved. When I go to the hot key field in the properties and press <ALT><SHIFT>S, it just clears, as if it is already in use.
Clear enough?